Story highlights Kelli Ward is challenging Flake in Arizona's 2018 Republican primary

Trump and Flake have feuded since early in Trump's presidential campaign

Washington (CNN) President Donald Trump turned hard on a fellow Republican Thursday, boosting the primary opponent of Arizona Sen. Jeff Flake and calling the incumbent "toxic."

"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.

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! — Donald J. Trump (@realDonaldTrump) August 17, 2017

Flake's team shot back in a statement issued later Thursday morning: "You don't serve Arizona by cutting backroom deals in Washington, D.C. That's why Senator Flake will always fight for the people of our state."

Flake's senior Arizona colleague, Sen. John McCain, also came to his defense on Twitter, apparently in response to Trump's tweet:

.@JeffFlake is a principled legislator & always does what's right for the people of #AZ. Our state needs his leadership now more than ever. — John McCain (@SenJohnMcCain) August 17, 2017

".@JeffFlake is a principled legislator & always does what's right for the people of #AZ. Our state needs his leadership now more than ever," McCain wrote.

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