Several outside groups supporting presumptive GOP presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE's campaign are struggling to meet fundraising goals, according to a Wednesday report.

The super-PACs, which are able to raise unlimited sums of money, have struggled to match past money operations or those supporting Democratic rival Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE.

NBC News reports that the main super-PAC supporting 2012 GOP nominee Mitt Romney, Restore our Future, had spent nearly $50 million at this point in the cycle.

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By comparison, three super-PACs supporting Trump — Great America PAC, Rebuilding America Now PAC and the Committee for American Sovereignty — have spent less than $4 million, according to Federal Election Commission (FEC) filing cited by NBC.

Ken McKay, New Jersey Gov. Chris Christie's (R) former campaign manager who is running the Rebuilding America Now group launched in early June, said $32 million in commitments from donors had not come through and the group has spent just $1.5 million on two TV ads.

Great America PAC, the oldest of the pro-Trump groups, has raised just $2.5 million since it was formed in early February. A co-founder of the group told NBC that the group doubled its fundraising the past month to $5 million, a third of its initial pre-Republican National Convention goal.

The Committee for American Sovereignty, run by former Ben Carson aide Doug Watts, expects to fall short of its $20 million preconvention goal. Watts said he expects fundraising to pick up after the convention.

Priorities USA, the main super-PAC backing Clinton, had raised $85 million through the end of May and has reserved $110 million in television advertising, according to NBC.

Trump's campaign itself has launched a full-court fundraising effort after posting dismal numbers for May, the month he was declared the presumptive Republican presidential nominee. He is expected to post higher number in June.