DES MOINES, Iowa – A super PAC started by Republican mega-donor Joe Ricketts is planning to spend at least $600,000 on an ad attacking Bernie Sanders’ liberalism that backers of Hillary Clinton believe is actually intended to support her Democratic rival.

The ad, which was first reported by the New York Times, ticks trough many of Sanders’ top agenda items, from free public college to higher taxes on Wall Street and the “super rich,” to free health care, before concluding that the Vermont senator is “too liberal for Iowa.” Since it’s basically impossible to be to “too liberal” for the Iowa Democratic caucuses, the ad immediately raised the suspicions of Clinton allies.

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“It looks like Bernie Sanders does in fact have establishment support. It’s just coming from the Republican establishment because they are afraid to run against Hillary Clinton,” said Guy Cecil, the top strategist for Priorities USA, a Democratic super PAC that backs Clinton.

Brian Baker, president of the ESA fund, the super PAC behind the ad, told MSNBC his group would likely spend even more than the initial $600,000 on digital and radio ads before next week’s critical caucuses in Iowa. “The Wall Street Journal says he has an $18 trillion spending program,” Baker said, citing Sanders’ rising poll numbers. ”It’s alarming.”

Clinton backers are not convinced. “I see you Joe Ricketts. And I know exactly what you’re up to,” tweeted Sen. Claire McCaskill, one of Clinton’s most vocal supporters, referencing the GOP donor behind the super PAC. “#ToddAkin Don’t fall for it Iowa Dems.”

In her 2012 Senate campaign, McCaskill spent $2 million attacking then-Rep. Akin as “too conservative” during his Republican Senate primary. McCaskill viewed Akin as the weakest possible opponent in the GOP field to face off against her, and he emerged as her opponent in the general election, which he ultimately lost.

Clinton allies have accused Republicans of exploiting a similar strategy to boost Sanders in the Democratic presidential primary, which they say is a clear sign their candidate is the stronger general election candidate. GOP super PACs have already spent at least $5 million attacking Clinton and $0 knocking Sanders.

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On Monday, the Republican National Committee blasted out an email to reporters suggesting Clinton’s support in the South was beginning to collapse after a South Carolina state representative switched his support from Clinton to Sanders.

“How big would you guess the collective eye roll is here every time we see someone parrot the Brooklyn talking point that we’re ‘rooting for Sanders?’” RNC spokesperson Michael Short said in an email.

Short insisted Republicans would happily run against either Sanders or Clinton. “We don’t care who we get, they’re both eminently beatable – one’s a gadfly socialist and the other has immensely bad favorability numbers and is facing a two-pronged FBI investigation,” he said. “[S]he’s just – for the moment, anyway – more likely to be the nominee so any challenges she faces we’re happy to point out.”

Correction: Due to an editing error, an original version of this story identified Baker as the donor behind the ad. Ricketts is the donor, Baker runs the super PAC funded by Ricketts.