Those advertisements touted state Sen. Erica Smith (D), a candidate who has raised about $210,000 for her campaign. Smith is the more liberal candidate in the field, openly embracing "Medicare for All" and the Green New Deal.

She is also an African American candidate running in a state with a primary on Super Tuesday, a day when the bulk of the North Carolina's Democratic presidential primary electorate will be African American.

But she is not the favorite of national Democrats, who back former state Sen. Cal Cunningham (D) as their strongest potential nominee against Tillis.

The FEC reports will show the Senate Leadership Fund spent about $3 million on television and radio spots boosting Smith, funneled through a group calling itself the Faith and Power PAC. The filings show the entirety of the Faith and Power PAC's contributions came from the Senate Leadership Fund.

“Who’s the Democrat for U.S. Senate endorsed by progressives and unions? Erica Smith. Who’s got the courage to vote for Medicare for All? Erica Smith. The number one supporter of the Green New Deal? Erica Smith again. Erica Smith is one of us,” a narrator says in the ad. “Vote Democrat Erica Smith for U.S. Senate, the only proven progressive.”

Even before the FEC filings, the Faith and Power PAC appeared to have ties to national Republicans.

In an interview, Steven Law, who heads the Senate Leadership Fund, said his group had decided to meddle in the Democratic primary after polls showed Cunningham running only narrowly ahead of Smith and after Democrats dumped millions of their own dollars into early advertising on Cunningham's behalf.

"The opportunity to engage in Democratic primaries is comparatively rare, because Democrats have managed to exert what I think is an admirable iron-fisted discipline over their primary fields. It's rare that you have a field where there's a potential for actual competition," Law told The Hill. "We had donors who were interested in taking on such a project."

One party meddling in another's primary is uncommon, but not unheard of. Democrats spent more than $2 million on behalf of two conservative candidates running against Tillis in the 2014 primary. Democrats also ran advertising that helped weak Republican candidates beat back stronger foes in states like Missouri and Nevada in recent years.

Democrats have been unusually active in North Carolina this year. Cunningham's campaign has spent more than $1.3 million on television spots so far. Outside groups including Vote Vets and an affiliate of the Senate Majority PAC have added about $10 million more, long before primary voters go to the polls. Vote Vets launched a major ad campaign in December, a move Law said piqued his interest.

"I think the tipping point for us was Vote Vets' multi-million dollar TV buy before Christmas last year. That signaled to us that Democrats themselves knew that Cunningham was dangerously weak and they felt the need to spend millions of dollars to prop him up. We thought that with a relatively small investment we could force them to spend a lot," he said. "The goal was less to change the ultimate outcome in the Democrat primary than to force the Democrats into a high-spending situation."

Several recent public polls have showed Cunningham pulling ahead of Smith, who represents a district in the Raleigh area. But those same polls show a huge percentage of Democratic voters remain undecided, potentially throwing into question a race that will feature high voter turnout because it will take place concurrently with the presidential primary.

“Republicans are in full-blown panic mode over Thom Tillis and the fact that they’re not even hiding it tells you everything you need to know about his chances in November,” said Rachel Irwin, a spokeswoman for the Democratic-aligned Senate Majority PAC.

Both men ultimately declined to run.