The ad was released the day following an attack ad from the Senate Conservatives Fund, which has put him in its sights for a primary challenge, charging that the senator "refuses to join the fight" to defund the law because he's declined to sign on to a letter circulated by conservatives that effectively pledges to shut down the government if the law isn't defunded.

Alexander's ad opens by declaring that the senator "has voted 23 times to get rid of Obamacare," and features a clip of a clash between him and President Obama at the White House Health Care Summit in 2010.

The two spar over whether the Democratic reform measure, then a bill in the Senate, would raise or lower premiums on families. While Alexander asserted a CBO report indicated the plan would raise premiums, Obama pushed back and declared the plan would actually decrease premiums, and an independent fact-checker ultimately declared that both were partially correct.

The ad is an indication that Alexander is taking the prospect of a primary challenge seriously. Though one contender, state Rep. Joe Carr (R), has already entered the race, SCF gave a lukewarm reception to the candidate and indicated he may have trouble gaining traction with some of the powerful outside groups a primary challenger would need to become competitive against Alexander.

Alexander has long been one of the GOP's more outspoken critics of ObamaCare. The attacks over his decision not to join the defund movement is an early taste of the coming GOP feud between the establishment and conservative wings of the party over how to handle the bill, expected to reach a head this fall.