Tens of thousands of mail ballots have been tossed out in this year’s primaries. What will happen in November?

More than 18,500 Floridians’ ballots were not counted during the March presidential primary after many arrived by mail after the deadline.

In Nevada, about 6,700 ballots were rejected in June because election officials could not verify voters' signatures.

And during Pennsylvania's primary last month, only state and court orders prevented tens of thousands of late-returned ballots from being disqualified.

As a resurgence in coronavirus cases portends another possible flood of absentee voting this fall, the issue of rejected ballots has emerged as a serious concern around the country, including in presidential battleground states and those with races that will decide control of the House and Senate.

While the number of rejected ballots in Florida and Nevada represents a fraction of those cast in their primaries, the unprecedented shift toward absentee voting during the coronavirus pandemic could make such margins potentially significant in the fall. In 2016, roughly 80,000 votes in Michigan, Pennsylvania and Wisconsin helped Donald Trump win the White House.

The rejection of ballots because of mail delays, signature match problems and errors in completing and sealing the forms could end up disenfranchising hundreds of thousands of people, voting rights advocates warn. It could also fuel doubts about the integrity of the 2020 vote, which Trump has already claimed without evidence will be "the greatest Rigged Election in history."

The growing risks has party officials and voting rights activists on high alert, raising the stakes for dozens of ongoing legal battles over absentee voting rules and placing additional pressure on election officials, whose staffs and budgets are already stretched thin by the demands of administering the vote during a pandemic.

"I think Kentucky's absentee ballot laws need an overhaul," said Fayette County Clerk Don Blevins Jr., a Democrat, after last month's primary, when roughly 7% of returned absentee ballots in his county, or 6,645, were rejected. "This election has really revealed just how strict we are. We could do better."

Worries about rejected ballots comes in the wake of the most chaotic presidential primary season in memory, when Americans fearful of the coronavirus embraced absentee voting in record numbers to avoid crowds at polling places. The result was an unexpected stress test of mail balloting systems, many of which were designed to handle only a small portion of the vote and not ready to scale up in response to a pandemic.

Election experts said that if the trend continues in the fall, the number of ballots that end up being tossed out could far exceed the nearly 319,000 mail and absentee ballots that were rejected nationwide in the 2016 general election, a rate of about 1% of mailed ballots, according to a survey by the U.S. Election Assistance Commission.

So far, there are signs that voting by mail is on track to reach new heights.

In New York, more than 1.7 million mail ballots were requested for the June primary, compared with about 115,000 that were submitted during the 2016 presidential primary.

The deluge overwhelmed election officials, who are still tabulating the results of some races. In the process, large numbers of mail ballots have been rejected because they arrived late, were missing a postmark or lacked a voter signature - as much as 19% in parts of Manhattan and Queens and 28% in parts of Brooklyn, according to preliminary tabulation figures obtained by The Washington Post.

A spokesperson for the New York City Board of Elections did not respond to a request for comment.

"If you project that forward to how many ballots they could receive in November, that's incredible," said Laura Ladd Bierman, executive director of the League of Women Voters of New York State.

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While a deep-blue state like New York is not in play for the presidential contest, the number of rejected ballots there could make a difference in more local elections.

In 2018, Democratic Assemblyman Anthony Brindisi flipped New York's 22nd congressional district by less than five thousand votes - a key victory that helped Democrats claim a majority of seats in the House.

"We are definitely not a battleground state, but we have a lot of other races that are key and could be close," said Bierman, noting that the state's ballot rejection rate was among the highest in the country over the past two election cycles. "They could have a big impact on how the House stands."

Mail ballots can be tossed for a range of reasons that vary from state to state.

If ballots are returned late, they are not counted unless a court intervenes. If they lack the required number of signatures from the voter, or if the voter's signature cannot be verified as authentic, the ballots are also rejected.

In some cases, the rules present a maze of obstacles and potential pitfalls.

If a state requires an "exact match" between signatures, even a tiny difference - such as a missing hyphen in a last name - can nullify a ballot. Ballots that arrive through the mail without a valid postmark might not be counted, even if the fault lies with the Postal Service. And some ballots that require a witness's signature are rejected if they do not also list that person's address.

The rules can be so strict in some jurisdictions that even a stray marking or small tear in the ballot envelope can cancel someone's vote.

Diane Haasch, a 69-year-old voter from Brookfield, Wis., was surprised to learn from a Washington Post reporter this week that her absentee ballot from the state's April presidential primary was not counted.

The cause? "Envelope damaged," according to a list of rejected ballots provided by local election officials.

"I know exactly what happened. I tore a little corner of it when I opened the envelope that it came in. I put a little piece of tape over it," Haasch said. "That is just really, really too bad. . . . Nothing was real close here where it would have made a difference, but it makes it possible to reject lots and lots of ballots over little problems."

Kristen Clarke, president and executive director of the national Lawyers' Committee for Civil Rights Under Law, a voting-rights advocacy group, called such rejections "grossly unfair."

"The use of inconsequential rules - for example, saying, 'We will not count this ballot because the voter provided the last two digits of their birth year instead of four' - and junk science-type rules, like signature-matching verification requirements, need to be set aside," Clarke said. "Those all provide avenues for officials to reject ballots from voters who are otherwise fully qualified."

Strict rules, such as those requiring that ballots be delivered instead of postmarked on or by Election Day, disproportionately affect young and minority voters, studies have shown.

Wendy Weiser, who directs the Democracy Program at the Brennan Center for Justice at the NYU School of Law, said those rules create obstacles for voters that are especially unfair during a public health crisis.

"Ballot rejection is very low hanging fruit," she said. "Election officials or states can just change their procedures to avoid these high rejection rates and ensure that every eligible voter has a fair shot at getting their ballots counted. . . . I think it's unconscionable not to make those changes at this point. Those are easy fixes."

The League of Women Voters is one of several groups pursuing legal action around the country to make mail balloting more voter-friendly. In New York state, the group filed suit last week to ensure that voters have a chance to "cure" their ballot, or address errors and deficiencies, before it is rejected.

Democratic elections lawyer Marc Elias is litigating in at least 15 states to establish what he calls the "four pillars" of fair absentee voting: allowing ballots to be postmarked on or before Election Day, providing government-paid return postage, permitting community organizations to help return sealed ballots, and establishing standards for the signature-matching process that election officials must follow.

Democrats argue that such measures are necessary to make sure that voters are not disenfranchised because of factors outside their control or arbitrary enforcement of the rules.

"This is why we are suing to ensure that every voter who votes by mail has their ballot counted," Elias tweeted Monday in response to a warning from a fellow attorney that votes cast by mail might not be counted. "Unfortunately the @GOP is fighting us at every turn."

Republicans, echoing President Trump, say that such moves will weaken election security and open the door to fraud.

A spokeswoman for the Republican National Committee said that attempts to "forcibly implement" policies that make it easier to vote absentee will "destroy public confidence in the integrity of our elections."

"Republicans want to make sure every valid vote is counted and our elections are free, fair, and transparent," RNC National press secretary Mandi Merritt said in a statement. "Democrats are pushing for a rushed transition to a nationwide vote by mail system that would result in more rejected ballots and an open door for fraud. States simply do not have the procedures in place to move to predominantly vote-by-mail elections or oversee the secure mass mailing of ballots to all registered voters by November."

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While some jurisdictions are scrambling to expand vote-by-mail for the first time this year, other communities - where large numbers of voters have already been using absentee ballots - have refined their protocols to keep ballot rejection rates low.

In Maricopa County, Ariz., which encompasses Phoenix and is the fourth-most populous county in the nation with 2.4 million registered voters, the rate of ballot rejection in the 2018 elections about 0.03%, or 307 out of roughly 1.2 million mailed ballots that were returned to the county, county officials said.

Rey Valenzuela, Maricopa County's director of elections services and early voting, said the county has improved its signature matching process since it began the use of no-excuse mail ballot in 1996. The use of mail-in ballots has expanded dramatically since, with 1.85 million of 2.4 million eligible voters in the county now on the permanent early voting list to receive their ballots by mail.

The county has multiple layers of verification and trains employees regularly on signature matching, including working with a forensics lab to learn the 23 points of a signature, including their slants and slopes. There are two layers of reviews by a lower-level employee and a managerial-level employee, and the county conducts a random audit on ballots to make sure the signatures match those on file. Updated technology allows officials to digitize signatures and keep them on file for the voter.

Election officials also regularly update their database to account for the evolution of voters' signatures. Voters are contacted by mail, email and phone if there is a problem with the match, and they can participate in a text-message service that updates them on the status of their ballot.

"We have had quite a few years of being able to perfect this process, how to indicate lessons learned, where we have been able to write best practices for other states," Valenzuela said.

But signature-matching rules have been a roadblock in other states where voting by mail is less common.

In Georgia, election officials in 2018 threw out 8,157 absentee ballots, or about 3% of the absentee ballots returned by mail for the election that November.

According to an analysis by the Atlanta Journal-Constitution, ballots cast by minority voters were twice as likely to be rejected.

The state's "exact match" law meant elections officials could require extra verification for the slightest inconsistencies, like a missing hyphen or an initial instead of a voter's full middle name. In some cases, voters' ballots have been canceled for administrative errors, such as providing two digits of a birth year rather than all four.

Voters then needed to verify their identity by providing additional information or resubmitting their ballots. But some voters were not notified in time to verify their identity before the required deadline, according to local news reports.

Earlier this year, the Georgia Democratic Party and the state settled a lawsuit stemming from the rejected ballots in the 2018 election. State election officials agreed to notify voters within three business days by email, phone and mail if their ballots were rejected, to give them ample time to provide additional information.

Offering a "cure" process for ballots that require more verification could lead to delays in tabulating the results of close races, as elections officials contact the voters to provide additional identifying information.

As they contend with the wave of mail ballots this year, election administrators said they find themselves in a tough spot - trying to simultaneously make voting more accessible and secure.

"It's a balancing act," said Aaron Ockerman, executive director of the Ohio Association of Election Officials. "We're not trying to make the forms intentionally difficult. It's a Catch-22."

Ockerman said confusing ballot instructions can lead voters to provide incorrect or incomplete information, such as listing their polling location address rather than their home address on mail-ballot application forms.

The Ohio secretary of state's office is now working with ballot design specialists to make its materials easier for voters to understand and fill out, he said.

The federal coronavirus relief bill passed in March set aside $400 million to help election administration, and local officials can use some of that money toward voter education efforts.

Kim Smith, deputy elections director for Defiance County, Ohio, said her office plans to use social media and its relationships with local news outlets to inform voters about the mail voting process - and how to avoid errors.

"We have a situation here where there is a unique stub on every ballot - if the voter tears that off, then we cannot count that ballot," she said. "So one of the things we're going to be doing is making it explicit on our ballots that they should not do that."

The Washington Post’s Amy Gardner and Jada Yuan contributed to this report.