President Donald Trump threw his support behind the primary challenger to Arizona Sen. Jeff Flake’s seat, criticizing Flake for being too weak on the border issue in a tweet Thursday morning.

“Great to see that Dr. Kelli Ward is running against Flake Jeff Flake, who is WEAK on borders, crime and a non-factor in Senate. He’s toxic!” Trump tweeted.

Ward, a physician and noted Trump supporter, is running against Flake for the Senate seat in the 2018 Republican Senate primary in Arizona. Ward is already set to bring on political operatives to her campaign that have connections to Trump.

“I, as well as others, are frustrated with Senator Flake’s weak style,” Ward told The Daily Caller News Foundation previously. “He hasn’t accomplished a thing in D.C., and his values do not align with the people of Arizona.”

Flake has been a staunch opponent of Trump’s travel ban, describing it as “morally repugnant.” He also released an excerpt of his book to Politico in which he said the Republican Party was in denial about Trump.

“I’ve been sympathetic to this impulse to denial, as one doesn’t ever want to believe that the government of the United States has been made dysfunctional at the highest levels, especially by the actions of one’s own party,” Flake wrote. “… For a conservative, that’s an awfully bitter pill to swallow.”

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