Smith, the beneficiary of Republicans' intervention, disavowed their support in an interview on Friday, telling POLITICO that "special interests and dark money should never play a role in an election." But, she added, the new disclosure made clear she had no connection to the GOP-funded group.

Faith and Power PAC's ads initially boosted Smith as the "only proven progressive" in the race when they began airing earlier this month. On Thursday, they launched an ad directly comparing Smith to Cunningham and questioning Cunningham's liberal credentials.

"We stole a page out of Chuck Schumer’s playbook, and it’s been more successful than we could have imagined," Steven Law, president of SLF and a former McConnell chief of staff and National Republican Senatorial Committee executive director, said in a statement. "Democrats are burning cash in a $13 million rescue mission for Cal Cunningham, who has proven to be a lackluster candidate with less money in the bank today than the beginning of the year."

Cal Cunningham is running in a five-person Democratic primary to face Republican Sen. Thom Tillis in November. | Chuck Burton, File/AP Photo

Democrats were spending heavily to boost Cunningham starting late last year, and their investments have increased substantially in the weeks leading up to the March 3 Super Tuesday primary.

Cunningham and his allies have worked to expose Faith and Power PAC's Republican ties in an effort to blunt the impact on the race, and they argue it shows that Republicans would prefer not to run against Cunningham in the fall. In a statement Friday after the disclosure about the group's funding, Cunningham said it was clear they "meddled in our state's election to try to mislead voters" and said it was because Tillis "has failed North Carolinians by every measure, and he's terrified to face me in November."

His campaign also launched a TV ad Friday morning in which he spoke directly to camera to push back against the ads, saying they are "deceptive ads attacking my values."

"They're paid for by Mitch McConnell's allies, and it's the very political corruption I'll go to Washington to fight," he said in the ad.

Tillis' campaign, meanwhile, hit Cunningham for calling Faith and Power PAC's ads "deceptive," and a spokesman claimed that he's backtracking and "embracing" a far-left agenda because the expensive primary took a "toll" on his campaign.

Smith has benefited from the nearly $3 million spent by the Republican group, which is the only TV advertising that has mentioned her name or candidacy. Her campaign reported Thursday having raised $45,000 so far this year and only $238,070 for the entire campaign, less than ten percent of the total fundraising by either Cunningham or Tillis.

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Smith defended her low-budget campaign in the interview, saying, "Every dime of that came from hard working North Carolinians who want a leader with integrity who doesn't have to be propped up with millions of dollars."

Smith, first elected to the state Senate in 2014, also called herself the most qualified Democrat in the race. "I have a record of distinction that is something that money cannot buy, and we are running off of our merit and off of our record of serving the people of North Carolina," she said.

Cunningham, a former one-term state senator who unsuccessfully ran for North Carolina's other Senate seat in 2010, has led in public polls, including a new poll this week showing his lead over Smith widening as early voting is underway less than two weeks before the primary. His campaign raised nearly $1.4 million so far this year and spent $1.6 million in the run-up to the primary, leaving him with $1.5 million on hand as of Feb. 12.

Cunningham's campaign has spent six figures on TV ads, and has also run nearly $1 million worth of ads using coordinated spending with the Democratic Senatorial Campaign Committee, which endorsed him, according to data from Advertising Analytics.

VoteVets, a group that backs Democratic veterans, has spent $7 million between its super PAC and an affiliated nonprofit on the race. And another super PAC, Carolina Blue — which has ties to national Democrats but has not yet reported the source of its funding — has spent nearly $4.5 million in the race to boost Cunningham, according to a new FEC filing.