As wealthy donors supporting Hillary Clinton pummel Donald Trump with negative ads, the Republican presidential candidate's own backers are struggling to return fire, hurt in part by a late start and conflicting signals from the campaign. In the latest setback, Thomas Barrack, a billionaire Los Angeles investor who said last month he'd gathered $32 million in commitments for a new super-political action committee supporting Trump, has decided not to donate to it or any other super-PAC, according to "Papa" Doug Manchester, a leading Trump fundraiser in California who has discussed the project with Barrack. The group raised only about $2 million through the end of June, according to Yahoo News.

"Tom Barrack and I just decided we would support the campaign to the maximum we can," Manchester said in an interview this week, referring to the roughly $450,000 maximum that an individual can donate to Trump and his party through a joint-fundraising committee. (A married couple can contribute $900,000.) That's a far cry from the multi-million-dollar checks that Barrack suggested last month were flowing to the super-PAC he was promoting, known as Rebuild America Now. Through a spokeswoman, Barrack declined to comment. 

Rebuild America Now is one of more than a half-dozen super-PACs jockeying for donors, so far without any emerging as the undisputed leader. As of the most recent reports filed with the Federal Election Commission, none had raised more than $3 million, while the main PAC supporting Clinton had amassed more than $88 million and is now spending around $4 million a week on anti-Trump messages. The groups are due to disclose updated figures over the next week.

Trump didn't begin actively soliciting funds for his campaign until May. And after winning the Republican primaries without major advertising campaigns, he has repeatedly expressed ambivalence about whether large-scale spending is even necessary. In private conversations with donors, top campaign officials have sometimes given conflicting signals about which super-PAC they prefer, according to people familiar with the matter who spoke on condition of anonymity. And Trump has sent mixed messages about whether he wants the support of super-PACs at all.

Earlier this year, the Las Vegas casino mogul Sheldon Adelson told Trump he'd give $100 million to support the presidential bid and explored forming his own super-PAC to direct the spending. But Adelson isn't currently pursuing his own super-PAC, according to Andrew Abboud, a top Adelson aide, confirming an earlier report in the Los Angeles Times. A spokesman for Adelson declined to comment on whether Adelson is sticking with the $100 million pledge.

The most recent super-PAC to enter the scene is Defeat Crooked Hillary, run by David Bossie, a longtime critic of the former secretary of state. The group is actually a repurposed PAC, originally formed by the New York hedge-fund manager Robert Mercer to support Senator Ted Cruz's presidential bid. The group had about $1.3 million left over at the end of May, and Bossie said that since then, Mercer made a "substantial" additional seven-figure contribution.

"You've seen what he's done in the past. He just doesn’t do things small," said Bossie. Mercer spent $13.5 million supporting Cruz and is already the second-biggest political donor of the 2016 election cycle, according to the Center for Responsive Politics.

Defeat Crooked Hillary started operations only last month and has disclosed just $20,000 in media spending since then. Bossie said he's still getting organized and will be meeting with donors and developing plans in the coming weeks. He said donors are interested in contributing, in part because the Trump campaign supports the effort. Mercer's seed money will cover all the overhead costs of running the group, so that "100 cents on the dollar" of outside contributions will go directly to anti-Clinton messages, Bossie said.

One of the oldest groups supporting Trump, formed in January and known as Great America PAC, probably won't meet a goal announced in May of raising $15 to 20 million by the Republican National Convention next week in Cleveland, said Eric Beach, a PAC official. He said it might be able to get close by the end of the month. It had gathered only $2.5 million as of May 31.

Beach said the group raised $2.5 million in June and another $1.6 million in the first 10 days of July, including donations of $250,000 from the family of Hank Seale, an Austin entrepreneur, and $150,000 from Doug Lebda, the founder of LendingTree in Charlotte, North Carolina. The group is investing in voter-turnout efforts in addition to pro-Trump TV ads.