(Bloomberg) -- Most attack ads against Donald Trump are convincing Republican primary voters to support the former president’s White House bid, a conservative political group has concluded.

The Win it Back PAC, a super political action committee that has spent millions of dollars to persuade GOP voters to support anybody but Trump, found that most of their tested messages were having the opposite of their intended effect, underscoring the challenge for the former president’s primary opponents to knock him off his perch as the Republican frontrunner. 

“All attempts to undermine his conservative credentials on specific issues were ineffective,” Win It Back PAC President David McIntosh said in a memo to donors this week after testing 40 different lines of attack. “This includes ads that primarily feature video of him saying liberal or stupid comments from his own mouth.”

As a result, the super PAC — which has spent $4 million in anti-Trump ads in Iowa and another $1.7 million in South Carolina — will pause further spending until another candidate can consolidate the anti-Trump vote. 

Trump is leading the his closest rival, Florida Governor Ron DeSantis, by 42 points in the FiveThirtyEight average of polls. DeSantis and six other challengers debated Wednesday without Trump, with a large field continuing to divide the half of Republican voters looking for a Trump alternative. 

McIntosh, who is also president of the conservative Club for Growth super PAC, said not every ad was a bust: The best lines of attack came from Republican voters who said they supported Trump previously but now question his ability to beat President Joe Biden. Republican primary voters also responded to ads noting Trump’s history of attacking other conservative leaders, and expressing “Trump fatigue” over his continued controversies. 

“The data shows we can and did move the needle,” McIntosh says. 

But even those messages showed diminishing returns. Although the ads reduced Trump’s favorability rating by up to 12 points, the support was split between other candidates and failed to point to a clear alternative.

Other groups that have run anti-Trump ads have had similar problems. The Republican Accountability Project ran ads arguing that Trump would lose to Biden, but ended it in August after finding it wasn’t effective enough.

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