President Trump’s increasing divide with congressional Republicans is expected to take center stage when he campaigns next week in Arizona, where outspoken Trump critic and GOP Sen. Jeff Flake is seeking re-election.

Flake’s opposition to Trump has increased in recent months, which has prompted the president to back a primary challenger to the first-term senator.

“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 Thursday. “He's toxic!”

The feud is part of a larger divide in which Trump is largely upset with GOP senators for failing to pass ObamaCare repeal-and-replace legislation, denying him a major legislative victory.

Trump is scheduled to campaign and hold a rally Tuesday in Phoenix.

Other GOP senators including Bob Corker of Tennessee, Dean Heller of Nevada and Majority Leader Mitch McConnell of Kentucky also have publically criticized Trump and as a result have become a target of his ire.

On Thursday, Flake, in a New York Times editorial, criticized the president’s immigration policy, although he didn’t specifically name Trump.

Corker, chairman of the Senate Foreign Relations Committee, on the same day delivered perhaps the strongest rebuke from congressional Republicans.

Trump has “not yet been able to demonstrate the stability, nor some of the competence that he needs to demonstrate in order to be successful,” Corker said, after Trump argued that “both sides” were responsible for the violence at a rally last weekend in Charlottesville, Va., that was organized by white supremacists and at which a counter-protester was killed.

McConnell, days earlier suggested publically that Trump had “excessive expectations” about swift passage of a repeal-and-replace measure, which prompted the president to chastise McConnell for failing to get the final votes in Congress.

“Couldn't get it done,” Trump tweeted earlier this week.

Flake, who also published a book last month questioning Trump's conservative values, says he mainly backs the president.

But he's shown he's willing to slam his party's leader despite knowing the president will hit back -- and hard.

Such spats illustrate the upside-down world of Republican politics heading into the 2018 elections.

Flake is loved by many high-ranking party officials and he is trying to hold onto a seat that the party needs to keep control of Congress, while Trump is actively campaigning against him.

The dynamic highlights the ongoing turmoil in the GOP over how closely to align with a president who remains unpopular with the general electorate but who still retains a devoted base of supporters, whom candidates like Flake will need to win.

Flake also said after Trump’s widely panned response to violence in Virginia that Republicans cannot claim to be the party of Lincoln “if we equivocate in condemning white supremacy."

Flake's colleague, Arizona Sen. John McCain, defended him, saying in a tweet that Flake is "a principled legislator & always does what's right for the people of #AZ. Our state needs his leadership now more than ever."

Trump then announced he was heading to the senators' home turf on Tuesday to rally his fans in Phoenix.

Flake's moves come with some risk in a state with a devoted base of Trump supporters. Still, a look at McCain's 2016 re-election bid can give him some solace.

McCain won his own primary election against Ward last year by more than 11 percentage points, and went on to beat the Democrat in the general election by 13 points, even after he broke with Trump in the month before the election and refused to vote for him.

Trump, meanwhile, won Arizona by fewer than 4 percentage points, a reminder of the state's moderate tilt.

Still, Trump got nearly 150,000 more votes than Flake did when he ran in 2012, a reminder of Trump's power to draw in new voters.

Republicans debating a break from the president must also be ready to weather a break from some of his voters.

Flake has been trying to walk a careful line. He's voted with Trump on important issues, but has been more willing than most GOP lawmakers to criticize him and did not endorse him last year.

"I think what Arizona expects of a senator is someone who will work with a president when he's right and oppose him when they think he's wrong," Flake told The Associated Press in an Aug. 10 interview. "That's what I'm doing and that's what I've done whether we've had a Republican president or a Democratic president."

Flake brushed aside criticism from Trump and declined to discuss Ward's campaign and emerging national profile, saying "I'll leave her to her campaign. I'll work mine."

Ward, a physician who drew only 39 percent of the vote in a primary fight against McCain last year, is seen as a longshot by top Arizona Republicans.

She hurt herself last month by urging McCain to step down following his brain cancer diagnosis and suggesting she should be considered to replace him.

Flake said at the time he was "dumbstruck" by her comments.

The Associated Press contributed to this report.