One of President Trump's biggest financial backers is donating big bucks to a super PAC working to defeat Sen. Jeff Flake, R-Ariz., in next year's election.

Robert Mercer, a hedge-fund billionaire who was one of Trump's top donors, is donating $300,000 to a super PAC backing former state Sen. Kelli Ward, according to Politico.

Ward is challenging Flake in next year's Republican primary in Arizona.

Flake was critical of Trump during the 2016 campaign, and the Arizona Republican recently re-upped his condemnations of the president as he promotes his new book, "Conscience of a Conservative." In an excerpt of the book published last week, Flake bemoaned the direction the GOP has taken since Trump assumed the presidency.

The Arizona Republican's opposition to Trump has earned the president's disapproval. During a White House press briefing last week, White House press secretary Sarah Sanders urged Flake to stop opposing Trump.

"I'm not sure about any potential funding of a campaign," Sanders said when asked if Trump would support a primary challenger to Flake. "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."

The White House has also met with Ward, as well as other Republicans who are considering challenging Flake in the Republican primary.

In addition to backing the super PAC supporting Ward, called KelliPAC, in the 2018 midterms, Mercer also donated to KelliPAC during last year's campaign. Then, Ward challenged Sen. John McCain, R-Ariz., who won.

"We are so grateful to Mr. Mercer for his courageous support for Kelli Ward, a true conservative champion. Early investments in a campaign like this are so valuable," Doug McKee, chairman of KelliPAC, said in a statement to Politico.

"Kelli is in prime position to carry out her message of accountable, constitutional government all the way to the United States Senate. Interest from additional donors is pouring in, and we are confident that leadership like Mr. Mercer's will allow us to run a robust winning effort all the way to November of 2018."