A number of super PACs supporting presumptive Republican nominee Donald Trump are struggling to meet expectations and fundraising goals, forcing them to retool their efforts with just four months until Election Day.

These groups – which have the ability to collect unlimited contributions from individuals, corporations and labor unions – have fallen well short of their expected goals of raising millions of dollars in the initial months of their formation. The controversies that have followed Trump since he effectively locked up the GOP nomination has been a major challenge, PAC officials say, and competition between the super PACs has further hindered the efforts.

At this point in the campaign cycle four years ago, the main super PAC backing Republican nominee Mitt Romney, Restore Our Future, had spent nearly $50 million. As of the end of May, three Trump super PACs, Great America PAC, Rebuilding America Now PAC, and the Committee for American Sovereignty, have spent less than $4 million, according to FEC filings.

Additionally, the three groups have raised a small fraction of the $60 million Restore Our Future had raised at this time last cycle. While Great America PAC is the only group to file monthly FEC disclosures that detail the organizations' fundraising and spending, officials of the other two groups have said they have fallen short of their fundraising goals.

Meanwhile, Priorities USA, the primary super PAC supporting presumptive Democratic nominee Hillary Clinton, has raised $85 million through the end of May and has reserved $110 million of television advertising.

Related: Clinton Outspends Trump In Battleground Ads, $26M to $0

Priorities USA has built in advantages. It was an existing super PAC that helped President Barack Obama’s reelection, and it has been working on Clinton’s behalf for more than a year. It quickly taped into Clinton’s fundraising machine and is on track to raise up to $200 million.

The super PACs backing Trump set high expectations but have been unable to meet them so far.

When Rebuilding America Now launched in early June, founder and California hedge fund investor Tom Barrack said he had $32 million in commitments. Nearly a month later, the group has spent just $1.5 million on two different television ads compared to the $110 million reserved by the pro-Clinton Priorities USA.

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Ken McKay, former campaign manager to New Jersey Governor Chris Christie’s presidential effort who is running the super PAC, said the $32 million has not materialized.

“All we can do is ask. And we are asking," McKay said of donors. "Any fundraising effort can be difficult and challenging but you can’t make people write checks.”

McKay said challenges include running against Priorities, a well-funded machine. Internal obstacles, however, include donors’ concerns that Trump can’t win in November. He also said potential donors say that Trump doesn't need their help because he has repeatedly said he can fund his own campaign.

McKay remains optimistic, noting, “We’re having some success and it just takes time.”

Depending on the level of fundraising success, the group plans on switching its television advertising to battleground states, a more expensive but more effective strategy, after the Republican convention.

The super PAC's struggles mirror the Trump's own challenges. He has implemented a fundraising apparatus with the held of the RNC that just got off the ground in June. While his fundraising numbers this month are expected to improve after a full-court press by his and the RNC's joint fundraising team, he has a lot of ground to make up against a well-funded Clinton campaign who had $26 million cash on hand at the end of May compared to Trump's $1 million.

Meanwhile, the oldest of the pro-Trump groups, Great America PAC, formed in early February, has raised just $2.5 million the four months since its inception.

Eric Beach, the co-founder of Great America PAC, sounds optimistic. He says that the group, which includes longtime Republican strategist Ed Rollins and former Trump field director Stuart Jolly, doubled its fundraising in the past month and that the June FEC filings will show that the group has raised an additional $2.5 million, bringing their total to $5 million – a third of its original goal ahead of the convention next month.

While Beach said fundraising, which targets both the $25 funder as well as the $25,000 donor, admitted that June is a slow month in the fundraising world and that he expects it to “multiply two or three times near the conventions” in July. Two fundraisers, one in California to be hosted by Mark Chapin Johnson, and one in Oregon are planned for early July.

“We’re not going to be on par with Hilary Clinton in terms of overall numbers, but as long as we are being efficient,” donors will keep supporting, Beach said.

The group is airing one national cable television advertisement, and Beach said he hopes to be on air “nonstop” through Labor Day.

A third super PAC, Committee for American Sovereignty, which is run by a former top aide to Ben Carson, Doug Watts, launched with the goal of raising $20 million by the convention. Watts said the group will come up “short of our pre-convention goal.”

The group is targeting voters, especially what they call “lost” voters, who are people registered to vote but have felt disaffected from the political process, as well as women and Hispanic small business owners. But he said the PAC is waiting until after the Republican convention – and maybe even after the Democratic convention – to implement its voter contact agenda.

“The commitments are there and I expect it to pick-up after the convention,” Watts added via email.

Seeing the challenges facing the existing super PACs, a fourth and the newest is taking a different route. The group, formed by Dave Bossie, the president of the conservative group Citizens United with the financial support of Republican donor Robert Mercer, says it’s goal is to focus on opposing the presumptive Democratic nominee rather than support Trump.

The Defeat Crooked Hillary PAC, which will be named Maker America Number One PAC for FEC purposes, plans to roll out a television ad this week. Its goal is to attract traditional Republican donors who are reluctant to spend money helping Trump.

“Some donors don’t want to associate with something overtly pro-Trump,” Bossie told Bloomberg. “This gives people an opportunity to aggressively get involved at whatever level they might want but have it solely focused on being a Hillary Clinton effort.”