The conservative Club for Growth is going after Sen. Mitt Romney Willard (Mitt) Mitt RomneyGOP set to release controversial Biden report McConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? MORE (R-Utah) in a new television ad, accusing the first-term senator and former GOP presidential nominee of siding with Democrats in President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE’s impeachment proceedings.

“There’s Mitt Romney threatening to vote with Democrats again to trot out spotlight-seeking blowhards who will trash President Trump on the witness stand,” a narrator says in the 30-second spot, which is set to begin airing on Fox News on Thursday in Salt Lake City and Washington, D.C.

The ad also accuses Romney of ignoring alleged wrongdoing by former Vice President Joe Biden Joe BidenBiden leads Trump by 36 points nationally among Latinos: poll GOP set to release controversial Biden report Can Donald Trump maintain new momentum until this November? MORE and his son Hunter. Trump and his allies have frequently brought up unsubstantiated allegations that Joe Biden pushed Ukraine to fire a prosecutor who had once investigated a firm that employed Hunter Biden as a board member.

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The ad from Club for Growth, a conservative outside group that has long advocated for lower taxes, comes amid a battle in the Senate over whether to allow witness testimony in Trump’s ongoing impeachment trial.

Republican leaders in the chamber have sought to block such testimony, while Romney and another GOP member, Sen. Susan Collins Susan Margaret CollinsMcConnell locks down key GOP votes in Supreme Court fight Most Americans think winner of election should pick next Supreme Court justice: poll Murkowski: Supreme Court nominee should not be taken up before election MORE (Maine), have said they will consider voting to allow witnesses. Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.) indicated on Tuesday that he did not yet have the votes to block witnesses.

The witness debate heated up this week after The New York Times reported that former White House national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE wrote in a forthcoming book that Trump had insisted the U.S. continue to withhold security aide from Ukraine until officials there helped with investigations into Democrats and the Bidens.

The account undermines a central claim of Trump’s impeachment defense. His lawyers have argued that the aid freeze was not related to the president's request for Ukraine to investigate the Bidens.

“Unfortunately, Mitt Romney continues to side with Democrats in their sham impeachment of President Trump, and voters deserve to know about his support of radical-liberals and their attempts to undo the results of the 2016 election and reverse the enormous economic gains achieved under Trump,” David McIntosh, the president of Club for Growth, said in a statement.

The Hill has reached out to Romney’s office for comment on the ad.

It’s not the first time Club for Growth has gone after Romney. An ad spot from the group last October accused Romney of plotting with Democrats to “take down” Trump with the impeachment process.