President TrumpDonald John TrumpSarah Huckabee Sanders becomes Fox News contributor The US-Iranian scuffle over a ship is a sideshow to events in the Gulf South Korea: US, North Korea to resume nuclear talks 'soon' MORE on Wednesday responded to an op-ed written by Sen.-elect Mitt RomneyWillard (Mitt) Mitt RomneyRomney: 'Putin and Kim Jong Un deserve a censure rather than flattery' A US-UK free trade agreement can hold the Kremlin to account Ex-CIA chief worries campaigns falling short on cybersecurity 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 FlakeAnti-gun violence organization endorses Kelly's Senate bid Arpaio considering running for former sheriff job after Trump pardon Overnight Energy: Warren edges past Sanders in poll of climate-focused voters | Carbon tax shows new signs of life | Greens fuming at Trump plans for development at Bears Ears monument 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 to award racing legend Roger Penske with Presidential Medal of Freedom Trump awards Presidential Medal of Freedom to economist, former Reagan adviser Arthur Laffer Second ex-Senate staffer charged in aiding doxxing of GOP senators 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.