Lt. Gov. John Fetterman raised $3.9 million in the first three months of his campaign for the U.S. Senate, his campaign said Thursday, setting a high bar in what’s likely to be a crowded and expensive Democratic primary.

Next year’s Pennsylvania Senate race is expected to be one of the most competitive in the country. Republican Sen. Pat Toomey isn’t seeking reelection, and the open seat offers Democrats perhaps their best chance to expand their razor-thin majority and help President Joe Biden pass his agenda.

The primary campaign is in its early stages, but Fetterman’s fund-raising haul represents an opening salvo to current and potential rivals that he will be tough to beat. State Rep. Malcolm Kenyatta, of Philadelphia, is the only other major declared candidate. A spokesperson for Kenyatta said the campaign wouldn’t be disclosing fund-raising totals yet.

State Sen. Sharif Street, of Philadelphia, filed paperwork Thursday with the Federal Election Commission and is expected to jump in soon, as is Montgomery County Commissioner Val Arkoosh. Pennsylvania Democrats anticipate members of Congress — including Reps. Chrissy Houlahan, Conor Lamb, and Madeleine Dean — might also run.

» READ MORE: Pennsylvania’s 2022 U.S. Senate race: Who’s in, who’s out and what comes next

Fetterman’s campaign said the $3.9 million was the most raised ever by a Pennsylvania Senate candidate during the first three months of a campaign. By comparison, in 2016 it took Democrat Katie McGinty eight months to raise $3.8 million before winning the primary with the support of the party establishment. The top three candidates in that Democratic primary election — McGinty, former U.S. Rep. Joe Sestak, and Fetterman — spent a total of about $9.5 million on the race.

McGinty’s general election against Toomey was the most expensive U.S. Senate race in the country that year, as the candidates and outside groups spent about $180 million, according to the Center for Responsive Politics.

Fetterman’s first-quarter haul is also among the biggest among Senate candidates across the country in recent years.

For example, Arizona Democrat Mark Kelly raised $4.1 million in early 2019 as the former astronaut laid the groundwork for his successful U.S. Senate campaign.

» READ MORE: The Braddock man John Fetterman confronted with a shotgun in 2013 says that should not stymie his Senate bid

Fetterman’s campaign said that it had received contributions from more than 90,000 people, and that 99% of donations were less than $200. That means Fetterman can go back to those donors repeatedly to raise more money; thousands have already contributed more than once, the campaign said. Individuals can contribute up to $2,900 to federal campaigns.

It wasn’t clear Thursday how much of his campaign cash came from donors who live in Pennsylvania. The campaign said it received contributions from people in all 67 counties.

Fetterman, the former longtime mayor of Braddock, in Allegheny County, was elected lieutenant governor in 2018. He’s used that platform to advocate for criminal justice reform and legalizing marijuana, and has built a considerable following among progressives.

It remains to be seen whether any other candidate in the field will be able to amass a similar network of small-dollar donors. Fetterman’s campaign, which has already spent hundreds of thousands of dollars on Facebook, said it had built an email list of one million supporters.

» READ MORE: Braddock’s mayor picks Malcolm Kenyatta over her predecessor, John Fetterman, in Pa. Senate race

“This election may be the most important race in the country in 2022,” said Rebecca Katz, a senior adviser to the campaign. “That’s why we are doing the work now to build up the broad coalition, infrastructure, and resources we will need to not only flip this U.S. Senate seat, but help Democrats across Pennsylvania win up and down the ballot in what could be a very tough year.”

“The Fetterman campaign is proving early that we will have the grassroots support we need to go the distance,” she added.

Fetterman’s full fund-raising report wasn’t available Thursday. Fund-raising reports for the quarter ending March 31 must be filed with the Federal Election Commission by April 15.

Staff writer Jonathan Tamari contributed to this article.