It's the group's third radio ad hitting incumbent senators over the issue. SCF on Tuesday launched ads targeting Sens. Lindsey GrahamLindsey Olin GrahamSenate GOP: Legislation to protect Mueller not needed Cornyn: Hearing on McCabe firing would be 'appropriate' McCain: Mueller must be allowed to finish investigation 'unimpeded' MORE (R-S.C.) and Richard BurrRichard Mauze BurrOvernight Cybersecurity: Trump-linked data firm Cambridge Analytica attracts scrutiny | House passes cyber response team bill | What to know about Russian cyberattacks on energy grid Week ahead: Senate Intel panel tackles election security Overnight Cybersecurity: Trump unveils new sanctions on Russia | Feds say Russian hackers targeted US energy grid | NSA nominee sails through second confirmation hearing MORE (R-N.C.) for their opposition to a threatened government shutdown over defunding ObamaCare.

The ad targeting Alexander is notable because he's been an outspoken opponent of the law.

The top Republican on the Senate Health, Education, Labor, and Health Committee, Alexander has previously called the law "a historic mistake" and repeatedly voted for its repeal and to delay its implementation.

But he wasn't one of a handful of Republicans to sign onto a letter circulated by Tea Party Sen. Mike LeeMichael (Mike) Shumway LeeSenate tees up Yemen vote for Tuesday Congress moving to end US involvement in Yemen This week: Congress races to prevent third shutdown MORE (R-Utah) pledging to oppose a government funding measure, and effectively shut down the government, if the measure doesn't include a provision defunding ObamaCare.

The SCF ad charges that "Republicans in Congress can stop ObamaCare by refusing to fund it, but Senator Lamar AlexanderAndrew (Lamar) Lamar AlexanderHouse leaves out ObamaCare fix from must-pass funding bill Overnight Health Care: Trump vows tougher borders to fight opioids | Senate considers vote to add ObamaCare fix to spending bill | Anti-abortion clinics take First Amendment case to high court Senate considers vote to add ObamaCare fix to spending bill MORE refuses to join the fight."

"Lamar Alexander voted to give President Obama money to implement ObamaCare earlier this year, and he'll do it again if you don't make your voice heard," a narrator ads.

The ad warns that the law is a "job killer" and a "liberal train wreck," and calls for listeners to sign a petition urging Alexander to join the defunding effort. It's backed by a $45,900 buy and will run on Tennessee radio for two weeks beginning Thursday.

Alexander looks well-poised for reelection, but SCF has long criticized the Senator and recently blasted him for his support of the immigration reform law that passed the Senate earlier this year.

He received his first primary challenger in state Rep. Joe Carr, who entered the race this week, but SCF's executive director, Matt Hoskins, indicated the group is unlikely to back him.

“If Carr doesn’t catch fire and a more compelling conservative enters the race, he could be the spoiler that helps re-elect Lamar Alexander. That’s something we want to avoid," he told The Tennessean.

The SCF ad is, however, an indication the group is still watching the race and looking for a stronger challenger.