Jeff Flake is one of the few elected Republicans to publicly break from Donald Trump, denouncing Trump-style populism in his new book and refusing to endorse Trump during the presidential election.

Now, Trump-supporting billionaire Robert Mercer is donating $300,000 to a super PAC backing Kelli Ward, one of Flake's Republican primary challengers, Politico's Alex Isenstadt reports.

Why it matters: Polls show that Flake is deeply unpopular in Arizona, and Republican primary voters are still overwhelmingly supportive of Trump. The precarious situation Flake finds himself in shows the risks other Republicans will be taking if they challenge Trump.