The anti-tax Club For Growth launched an attack ad against Sen. Mark Pryor (D-Ark.) Thursday, casting him as President Obama's "best ally in Arkansas."

"He's the only Arkansan in Congress today who voted for Obamacare. The only one who voted for the Obama stimulus," says the narrator of the spot, which the Club says is backed by a six-figure buy.

Pryor is one of the most vulnerable Senate Democrats up for reelection next year. On the Republican side, Lt. Gov. Mark Darr appears set to challenge the senator. Another name to watch is freshman Rep. Tom Cotton (R), an Iraq war veteran whom the Club endorsed last cycle.

When asked if the Club's ad was meant to entice Cotton to run, a Club spokesman only responded that the group has supported him before, and would seriously consider supporting him as a Senate candidate.

"Our PAC supported Tom Cotton’s run for Congress last year and so far he’s been every bit the limited-government pro-growth leader we thought he’d be," said Club spokesman Barney Keller. "The Club for Growth PAC would strongly consider supporting him if he chose to try and take his free-market principles to the United States Senate, but as far as we know he hasn’t made a decision yet."

Pryor is set to formally launch his reelection bid next month with some help from Bill Clinton.