Hedge fund owner and Trump ally Robert Mercer is donating $300,000 to support a primary opponent of Trump critic Republican Arizona Sen. Jeff Flake, according to a Politico report published Wednesday.

The donation comes after rough criticism of President Trump by Flake, a longtime opponent of the president. The Arizona senator recently came out with “Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle,” a book that attacks Trump for not being a conservative and claims the GOP is in “denial” about the president.

Mercer was a major donor to Trump’s presidential campaign and reportedly helped get both Steve Bannon and Kellyanne Conway jobs on the Trump campaign. The New York hedge fund owner will donate $300,000 to a Kelli PAC, a super PAC supporting former state Sen. Kelli Ward. Mercer and his wife Diana donated $700,000 to this PAC in 2016 when Ward faced off against another Trump critic Sen. John McCain.

White House spokeswoman Sarah Huckabee Sanders wouldn’t deny last week the possibility that Trump would support a primary challenge to Flake.

“I’m not sure about any potential funding of a campaign,” Sanders said. “But I think that Sen. Flake would serve his constituents much better if he was less focused on writing a book and attacking the president.”