The Club for Growth will go up with a sizable ad buy in the competitive Wisconsin Senate GOP primary Friday, according to sources with knowledge of the race.

The club endorsed former Rep. Mark Neumann (R-Wis.) in September and has attacked former Gov. Tommy Thompson, who began the Senate race as the frontrunner due to his high name identification, from the right.

But businessman Eric Hovde has emerged as the primary conservative challenger to Thompson in recent weeks despite the club’s endorsement of Neumann. Recent polls have shown Hovde and Thompson breaking away from the rest of the GOP primary pack. Hovde, who has never before served in public office, has spent millions of his own money to introduce himself to Wisconsin voters.

It is unclear what the content of the Club for Growth ad is, and whether it targets Thompson or both Thompson and Hovde. It’s unlikely the spot is a positive ad highlighting Neumann’s record.

The primary is Aug. 14. The winner will face Rep. Tammy Baldwin (D) in what is expected to be a hard-fought general election this fall.