President Trump targeted Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) in a tweet Thursday morning calling the senator weak on immigration and crime, while backing Flake's primary opponent.

"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 said.

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

ADVERTISEMENT

The president has made no secret about his dislike of Flake, who is a vocal critic of Trump's, and has reportedly floated the prospect of spending millions to unseat him. Flake is currently in the midst of a tour for his new book, which argues that the GOP should take a stronger stance against Trump.

Ward, a former state senator who mounted an unsuccessful primary challenge against Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) last year, is Flake's only primary opponent so far. She's challenging him from the right, bashing him for his criticisms of the president.

McCain, himself a target of Trump's ire over his vote on GOP healthcare legislation, defended his Arizona colleague on Twitter later Thursday morning.

.@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

The White House has yet to endorse a primary opponent to Flake, but Trump's tweet is a major boost to Ward.

Trump visited Arizona often during last year's presidential election, holding seven rallies in which he pledged a crackdown on illegal immigration and the construction of a wall on the southern border. He is scheduled to hold a rally in the state next week, providing an opportunity for him to criticize Flake and fellow GOP Sen. John McCain (Ariz.) if he chooses.

Flake has not backed down from his criticism of Trump, slamming the president this week for blaming “both sides” for the violence after white supremacist groups clashed with counterprotesters in Charlottesville, Va., over the weekend.

“We can't accept excuses for white supremacy & acts of domestic terrorism. We must condemn. Period,” Flake tweeted Tuesday.

We can't accept excuses for white supremacy & acts of domestic terrorism. We must condemn. Period. — Jeff Flake (@JeffFlake) August 15, 2017

During the 2016 campaign, Flake refused to support Trump.

The signal from the White House will undoubtedly concern top Washington Republicans, who are fearful Trump’s intervention in a primary could lead to a weaker GOP candidate running in next year's midterms elections.

Republicans have a two-seat majority in the Senate.

Trump allies have already begun to circle the wagons around Ward. Conservative mega-donor and Trump ally Robert Mercer donated $300,000 to a super PAC supporting Ward, and former Trump campaign hands have signed onto her campaign.

Other candidates could jump in, including state treasurer Jeff DeWitt and former Arizona GOP Chairman Robert Graham.

All three have spoken with White House ahead of their potential bids.

The increased activity in Flake's primary has been a boon for Democrats mounting their own challenge. Rep. Kyrsten Sinema (D-Ariz.) continues to mull a bid, and both Democrats and Republicans agree that she could pose a strong challenge to Flake even if he survives a primary.