The ominous mailer has an image of Doug Mastriano on one side. The other has a clipping from a news article reporting that the state senator wanted the federal government to suspend privacy laws so local health departments could “release names and locations” of COVID-19 patients.

“We can’t trust Doug Mastriano,” warns the mailer, which was paid for by a well-funded conservative political group and sent to Republican primary voters this week.

The attack was the clearest sign yet that most Pennsylvania Republican insiders see Mastriano, one of the state’s leading election deniers, as the front-runner for the party’s nomination for governor — and are increasingly worried about it.

Less than three weeks before the May 17 primary election, polls and interviews with campaign advisers and other GOP insiders suggest the nine-candidate race remains a tossup. The share of voters who say they are undecided — as high as 40%, according to an April Franklin & Marshall College survey — is unusually high this late in the campaign. That might be partly because it’s the biggest GOP gubernatorial primary field since 1978, when Dick Thornburgh won with just 33% of the vote.

Campaign aides say this year’s winner could prevail with as little as 28%.

Voters this year have endured a deluge of TV ads, many of them negative, especially in Pennsylvania’s Republican Senate primary, the most expensive in the country. That’s likely making it harder to get voters’ attention in the homestretch — and to change the dynamic of the race.

Mastriano and former U.S. Rep. Lou Barletta have led public polls for most of the campaign.

Former Delaware County Councilman Dave White and former U.S. Attorney Bill McSwain are also in the hunt. White met with former President Donald Trump, whose endorsement in the race is highly coveted, at his Mar-a-Lago resort in Florida on Tuesday, according to three people familiar with the matter.

And Trump’s declaration of political war against McSwain earlier this month — the former president called him a “coward” for not prosecuting voter fraud in 2020 — hasn’t knocked McSwain out of the race. On the contrary, McSwain is far outspending his rivals on television in the final weeks.

State Senate leader Jake Corman, who almost dropped out of the race before Trump apparently persuaded him to stay in, has struggled to compete with the top tier.

“It seems to be coming down to Doug Mastriano or myself. It’s a close race,” Barletta said Thursday during a meet-and-greet event near the Poconos. Trump’s endorsement, he said, “can definitely swing an election.”

» READ MORE: 4 takeaways from this week's Pennsylvania Republican governor’s debate

As a debate on Wednesday showed, the leading candidates largely agree on the issues. They say Democratic Gov. Tom Wolf, who is term-limited, badly mismanaged the pandemic, harming businesses and infringing on personal freedom. They want to eliminate no-excuse mail voting, enact new abortion regulations, drill for more natural gas, lower taxes, and expand access to charter schools and other alternatives to traditional public schools.

So the campaign mostly hinges on which candidate voters know the most and trust the most. And Mastriano is leading the pack, according to recent public polls and internal GOP surveys.

Mastriano, a retired Army colonel elected to the state Senate in 2019, has already outperformed the expectations of some who dismissed him as a fringe candidate with a small core of supporters. Some in the GOP now worry the party blew its chance to coalesce behind an alternative candidate who would stand a better chance of beating presumptive Democratic nominee Josh Shapiro, the state attorney general, in the general election.

He played a leading role in efforts to overturn Trump’s 2020 defeat in Pennsylvania, and party leaders worry his penchant for associating with fringe groups — like his decision to speak at an April event promoting the QAnon conspiracy theory — could jeopardize Republican majorities in the state legislature. The congressional committee investigating the Jan. 6, 2021, attack on the U.S. Capitol subpoenaed Mastriano in March for documents and testimony about his involvement in efforts to overturn the election.

Mastriano said he went to the “Patriots Arise” event to speak with his constituents and described news coverage as unfair. “People across the state are sick and tired of being labeled something because you disagree with them politically,” he said during the debate.

The state GOP declined to endorse a candidate at its winter meeting. “Now it’s coming home to roost,” said a Republican insider with close ties to party leadership.

A well-funded campaign to stop Mastriano is now underway. The question is whether it’s too little, too late.

Commonwealth Leaders Fund, a conservative group that has spent about $6 million boosting McSwain’s campaign, paid for the mailers criticizing Mastriano’s March 2020 statements. The group declined to say how much money it’s committing to the effort. McSwain’s campaign is also attacking Mastriano on the issue in TV ads.

When McSwain brought it up at the debate, Mastriano replied: “Nonsense is still nonsense, especially when it’s spoken from an attorney.”

“Josh Shapiro is rooting for Doug Mastriano and Teddy Daniels to win the Republican primary. They are his best shot at winning in November,” said Matt Brouillette, the PAC’s treasurer, referring to Mastriano’s chosen running mate for lieutenant governor, whose wife has accused him of domestic abuse.

And on Friday, a new group called Pennsylvania Patriots for Election Integrity started airing in Harrisburg, Johnstown, and other markets a quarter-million dollars’ worth of TV ads attacking Mastriano. Even if the attacks work, it remains to be seen which candidate would benefit the most from Mastriano’s decline.

Four leading contenders

McSwain, a West Chester native running as a “conservative outsider,” started rising in the polls in March and April thanks to millions of dollars in TV ads paid for by a group bankrolled by the billionaire Pennsylvania investor Jeffrey Yass.

He’s facing headwinds from Trump’s criticism – something the former president will almost surely bring up when he holds a rally in Westmoreland County next week with Mehmet Oz, his endorsed Senate candidate. But McSwain has the most cash in the race and has booked $3.4 million worth of TV ads through the primary, according to AdImpact, which tracks political advertising. That’s even more than the uber-rich Senate candidates Oz and David McCormick are set to spend over that period.

That means undecided voters who are just starting to tune in might hear more about McSwain than anyone else. Even Republicans unaffiliated with McSwain’s campaign say he still has a path to victory.

Barletta’s team is betting that his high name recognition and strong favorability ratings among primary voters, built over a two-decade political career that included a 2018 Senate run, will make him an attractive choice both to undecided voters and to Mastriano supporters who might sour on him.

“I am proven, road-tested, and ready,” Barletta said during the debate.

And White’s advisers say his background as a businessman and third-generation steamfitter is resonating in a Republican Party that wants to appeal to blue-collar workers. White, who has lent his campaign $4 million, trails only McSwain in television advertising.

Three of the four Republican county parties in the Philadelphia suburbs have endorsed White — a sign of a late effort to rally behind a single alternative to Mastriano. But even that prompted some backlash, as some activists in Bucks County objected to the endorsement process, prompting party chair Pat Poprik to issue a mea culpa over the process.

» READ MORE: Everything you need to know about voting in Pennsylvania's May 2022 primary election

For his part, Mastriano is running an unconventional but well-organized campaign — as demonstrated by the unrivaled 28,000 signatures his team collected to qualify for the ballot and yard signs seen across the state, even in more moderate suburban towns. He entered the race better known than all of his rivals except Barletta, and just started airing his first television ad this week.

Even his detractors say Mastriano has tapped into anger animating the Republican base over the pandemic and the 2020 election.

“We need somebody who’s not afraid to stand up to the left,” Mastriano said during the debate.

Advisers to other candidates have long argued that Mastriano hasn’t tried to expand his base of support. As one put it, anyone attending the “Patriots Arise” event in Gettysburg this month that featured a video promoting QAnon was probably already in Mastriano’s corner. During the debate, he lashed out at “the McCarthyist Democrat cabal” in Washington — a line that may please his core supporters but that others might find strange.

Trump looms

Looming over all of this is Trump himself.

In such a close race, his endorsement could prove decisive. But Trump likes multiple candidates, according to people familiar with the matter, and no one knows what he will do. White’s trip to Mar-a-Lago — where he was accompanied by pollster John McLaughlin, who has also worked for Trump — came weeks after Barletta held a fund-raiser there.

Mastriano has campaigned with current and former Trump aides and has loyal supporters in the broader MAGA orbit.

Corman has golfed with Trump and stayed in touch, but he has struggled to gain traction in the polls. He’s airing an ad featuring former Trump adviser Kellyanne Conway describing Corman as “the true America First fighter.”

“It’s not an easy bar to clear if you’re seeking his endorsement,” said one person familiar with the process. Since Trump likes several candidates, “you don’t want to trash his friends.”

-Staff writer Anna Orso contributed to this article.