President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE lashed out at Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeHow fast population growth made Arizona a swing state Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden MORE on Thursday, questioning how the retiring Arizona Republican could even "think about running for office" at any level in the future.

In a tweet, the president accused Flake, a frequent Trump critic, of being "humiliatingly forced out of his own Senate seat," and attacked speculation about the senator's future plans for public office.

How could Jeff Flake, who is setting record low polling numbers in Arizona and was therefore humiliatingly forced out of his own Senate seat without even a fight (and who doesn’t have a clue), think about running for office, even a lower one, again? Let’s face it, he’s a Flake! — Donald J. Trump (@realDonaldTrump) June 7, 2018

Flake attacked Trump's recent moves on trade in a Senate floor speech on Thursday, calling his tariffs on major allies "not grown-up leadership," according to excerpts released a day earlier.

“What shall our friends make of such erratic behavior? How will they respond to such confusing actions? And, most importantly, how long will they remain our friends if this irrational approach continues?” Flake's speech reads.

Flake announced he would retire last year amid a potentially challenging reelection bid, and has not ruled out the possibility of a run to challenge Trump for the GOP nomination in 2020.

"It’s not in my plans but I have not ruled anything out,” Flake said of running for president on NBC's “Meet The Press” in May.

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The senator faces underwater approval ratings at home, however, where 49 percent of Arizona residents disapprove of the job he has done in Washington compared to 32 percent who approve, according to a January Morning Consult poll.

Trump and Flake have frequently and publicly battled over policy and conduct in the White House since Flake announced his resignation last year, with Flake accusing Trump of practicing the politics of "anger and resentment" in a Senate floor speech last year.

“We must never regard as normal the regular and casual undermining of our democratic norms and ideals,” Flake said. "We must never meekly accept the daily sundering of our country, the personal attacks, the threats against principles, freedoms and institution, the flagrant disregard for truth and decency, the reckless provocations — most often for the pettiest and most personal reasons, reasons having nothing to whatsoever to do with the fortunes of the people that we have been elected to serve."