President Donald Trump on Thursday tweeted his enthusiasm for Arizona Senate candidate Dr. Kelli Ward and her bid to challenge incumbent Republican Sen. Jeff Flake's seat in the 2018 GOP primary. Flake has become one of Trump's largest Republican detractors.

“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,” Trump tweeted early Thursday morning. “He’s 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!

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

Flake refused to endorse Trump during the 2016 presidential elections and called on then-candidate Trump to drop out of the race in October after an audio recording of Trump claiming to "grab" women "by the p***y" was released to the public.

The senator's disdain for Trump has not abated over time. In his recent book, "Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle," Flake wrote of Trump's ascension that “we pretended that the emperor wasn’t naked,” and added “even worse: We checked our critical faculties at the door and pretended that the emperor was making sense.”

Flake's opposition greatly angered Trump, and in July, the president offered to pay $10 million of his own money to assist in unseating Flake, according to The Independent.

Earlier this month, hedge fund manager and Donald Trump supporter Robert Mercer donated $300,000 to KelliPAC, a super-PAC created by Trump supporters for the purpose of helping Ward defeat Flake in 2018.