The super PAC putting its weight behind the presumptive Democratic presidential nominee Hillary Clinton raised $12.1 million in May, its largest monthly fundraising take of the election cycle as the U.S. White House contest shifts toward the general election.

A representative of Priorities USA Action, founded in 2011 to support Democratic Party candidates through contributions by rich benefactors, corporations and unions, told the Wall Street Journal Monday it had $51.9 million in its coffers and an additional $45 million in commitments at the end of last month, far more than any political action committee supporting the former U.S. secretary of state’s expected opponent, presumptive Republican presidential nominee Donald Trump.

Trump, who fired campaign manager Corey Lewandowski Monday, trails his rival in the fundraising race as prominent GOP members have kept their distance from the billionaire businessman, either denouncing some of his remarks as racist or doling out tepid, arm’s-length endorsements. Some prominent Republicans appear to be abandoning the presidential election altogether, as they seem to be attempting to ensure their party holds or takes control at the state level.

Priorities USA Action has raised $87.9 million in this election cycle while supporting Clinton. It has reserved about $147 million in TV, radio and digital advertising airtime to date, which far outpaces Trump’s ad spending. He has spent virtually nothing on advertising, relying instead on his attention-grabbing incendiary speeches that have garnered the support of millions of GOP voters, but haven’t played well outside the party.

Trump’s campaign has been largely self-funded through loans he gave it. But as he begins to rely on campaign contributions, he’s under pressure to ensure he doesn’t use those donations to pay back the loans. The candidate said May 12 he would not use these contributions to pay back the roughly $40 million he’s lent his campaign. But he has yet to file paperwork with the Federal Election Commission indicating that he has forgiven the loans.

“Trump assured reporters that he would not use donor money to pay himself back,” Charlie Spies, former election law counsel for the Republican National Committee, wrote in the Hill Monday. “Now, they will be watching to see if he officially notified the FEC that he forgave his loans. Trump could have filed a notification with the FEC at any point to demonstrate this, but, to date, he has not.”

Trump will need to step up his ad spending between now and the general election to lure supporters outside his base in the GOP. He currently trails Clinton by 6 percentage points in the RealClearPolitics average of all general election polling data. He had been virtually tied with her May 23.

The Great America PAC, one of the earliest pro-Trump’s fundraising groups, announced Monday it would spend $700,000 on television ads in the wake of the June 12 mass shooting at the Pulse nightclub in Orlando, Florida, to publicize the candidate’s stance on acts of terrorism.

Meanwhile, Clinton’s campaign raised $240 million through May and had $42 million on hand at the end of the month, according to the FEC.

Zika-Virus-Vaccine-Update-Hillary-Clinton-Congress-Funding-Billion Hillary Clinton will reportedly call on Congress to come back in session and pass legislation to provide emergency funding for the Zika virus. Photo: Chip Somodevilla/Getty Images