What seems like a generic campaign ad pitching Erica Smith, a North Carolina state senator, as “the only proven progressive” in the state’s high-profile Senate race is actually part of a multimillion dollar investment from a mysterious super PAC — the innocuously named “Faith and Power PAC” — with apparent ties to Republicans.

The ad campaign, which began last week ahead of the March 3 primary, immediately disrupted the bid from frontrunner and Democratic leadership favorite Cal Cunningham to emerge from his primary and face incumbent GOP Sen. Thom Tillis in November.

The North Carolina race is critical: Without beating Tillis, Democrats’ path back to the Senate majority is nearly impossible. Cunningham, a former state lawmaker and military veteran, lost a Senate primary in 2010, and Democrats are eager to avoid the same result this year. But things are getting messy — and expensive…

Privately, Senate Democrats have been discussing the matter internally, with one fretting that Smith is “unelectable” in a general election and will be painted as a Bernie Sanders (I-Vt.) acolyte.