Donald Trump’s fundraising is slipping far behind Hillary Clinton’s as the candidates tilt toward a general election battle – though the latest financial reports also show both presumptive presidential nominees raising well below what their 2012 counterparts had at this stage in the race four years ago.

The latest filings underscore a potential enthusiasm gap on both sides. David Avella, chairman of Republican recruiter GOPAC, said the candidates are facing fundraising headwinds on two fronts.

“First, the economy continues to muddle along and that has had impact on some donors in getting engaged,” he told FoxNews.com. “And, given the high unfavorable ratings of both candidates, it’s not surprising that donors are slower investing than in the past.”

Still, Clinton is vastly outraising Trump right now.

The reports show donors gave just over $3 million to Trump's campaign in May, while the billionaire businessman lent his effort another $2.2 million.

Clinton's campaign raised more than $26 million in May, her report shows.

The biggest difference is in cash on hand.

Trump’s campaign started June with $1.3 million in the bank. Clinton’s started with $42 million.

The figures revived questions Tuesday about how Trump is funding his campaign and about his self-proclaimed wealth. Billionaire investor Mark Cuban zinged him on Twitter:

Trump downplayed the fundraising gap in an interview with Fox News, on the heels of his decision to fire campaign manager Corey Lewandowski.

"We want to keep it lean. I'm not looking to spend all this money. She's going to spend more than $1 billion,” he said.

At the same time, he suggested his tensions with Republican leaders were holding his operation back. He said the Republican National Committee and its chairman, Reince Priebus, "have been terrific," but "it would be nice to have full verbal support from people in office."

The presumptive Republican presidential nominee continues to face criticism from Republican leaders, including House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell. Although both have endorsed him, last week they condemned Trump's renewed call to impose a temporary ban on foreign Muslims entering the country.

On both sides, past Federal Election Commission reports show Trump and Clinton are still off pace when compared with their 2012 counterparts.

In May of 2012, Mitt Romney’s campaign reported raising $23 million and ending the month with $17 million in the bank.

President Obama’s juggernaut raised $39 million and started June with nearly $110 million.

In 2008, Arizona Sen. John McCain raised $21 million in May, and ended the month with over $31 million on hand. Obama raised $23 million in that period, and had more than $43 million on hand.

Avella acknowledged, though, that Trump’s primary success this year on a shoestring budget shows he’s “embarked on a new model.”

The X-factor for Trump is to what extent he dips into his own private wealth. Throughout the primaries, Trump boasted that his campaign was largely self-funded. As he pivoted to the general election, he reassessed and opened the door to raising money from donors – for his campaign as well as for the Republican Party.

On NBC's "Today," Trump said he may have to tap more into his own funds. "If it gets to a point, I'll do what I did in the primaries. I spent $55 million in the primaries. I may do it again in the general election, but it would be nice to have some help from the party," he said.

Whether he raises from outside sources or pays out of pocket, those expenses must be reported to the FEC.

As both Trump and Clinton charge into their respective nominating conventions next month, Trump’s decision to fire Lewandowski could signal a renewed fundraising commitment.

Fox News has learned that Trump is hosting a 48-hour online fundraising drive in which he’ll be personally matching donations up to $2 million. The campaign says they want to encourage small-dollar donations, of anywhere between $1 and $1,000.

With Lewandowski gone, campaign chairman Paul Manafort is in charge of the whole operation. Manafort previously defended the campaign’s decision to raise money.

Back in May, he told “Fox News Sunday:” “The Democrats have said they're going to be spending hundreds of millions of dollars to try and spread lies about Donald Trump and the Republican Party. Trump has said to compete against them he will support the party and the party's efforts.”

FoxNews.com’s Liz Torrey and Judson Berger and Fox News’ John Roberts and The Associated Press contributed to this report.