President TrumpDonald John TrumpDemocratic senator rips Trump's 'let them fight' remarks: 'Enough is enough' Warren warns Facebook may help reelect Trump 'and profit off of it' Trump touts Turkey cease-fire: 'Sometimes you have to let them fight' MORE on Wednesday responded to an op-ed written by Sen.-elect Mitt RomneyWillard (Mitt) Mitt RomneyPelosi, Schumer hit 'flailing' Trump over 'sham ceasefire' deal Overnight Defense — Presented by Boeing — Pence says Turkey agrees to ceasefire | Senators vow to move forward with Turkey sanctions | Mulvaney walks back comments tying Ukraine aid to 2016 probe On The Money: Senate fails to override Trump veto over border emergency | Trump resort to host G-7 next year | Senators to push Turkey sanctions despite ceasefire | McConnell tees up funding votes MORE (R-Utah), suggesting the incoming senator should be more supportive of his policies.

In a tweet, the president called on Romney to "focus on Border Security and so many other things where he can be helpful," while questioning whether he will be "a Flake," referencing Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeTrump's GOP impeachment firewall holds strong How to survive an impeachment Are Senate Republicans certain that Trump can return to office? MORE (R-Ariz.), a frequent Trump critic who did not run for reelection last year.

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"Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not," Trump wrote Wednesday morning.

"Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful. I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!" he added.

The president's tweet came hours after Romney targeted him directly in a Washington Post op-ed, accusing him of a glaring "shortfall" in terms of character and leadership abilities.

"With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable. And it is in this province where the incumbent’s shortfall has been most glaring," Romney wrote Tuesday.

"I do not intend to comment on every tweet or fault. But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions," he continued.

Trump later doubled down on his comments during a Cabinet meeting at the White House. 

“I wish Mitt could be more of a team player. I endorsed him and he thanked me profusely,” Trump said. 

If Romney he said if he fought as hard against President Obama as he did against him, Trump said “he would have won the election.”

Elected in November to succeed the retiring Sen. Orrin HatchOrrin Grant HatchTrump holds more Medal of Freedom ceremonies than predecessors but awards fewer medals Trump to award Medal of Freedom to former Attorney General Edwin Meese Trump to award racing legend Roger Penske with Presidential Medal of Freedom MORE (R-Utah), Romney is due to be sworn in as Utah's junior senator on Thursday.

Despite being a frequent critic of Trump's during the 2016 election, Romney thanked Trump for his support during his 2018 Senate run. He was briefly considered to be in the running to serve as Trump's secretary of State in 2017.

Jordan Fabian contributed.

This story was updated at 1:30 p.m.