Washington (CNN) President Donald Trump fired back at Mitt Romney after the Republican senator-elect penned an op-ed saying Trump "has not risen to the mantle of the office."

"Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not," Trump tweeted, referring to retiring Arizona Sen. Jeff Flake, a Republican who has criticized the President often in the last few years. "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!"

Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not. 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! — Donald J. Trump (@realDonaldTrump) January 2, 2019

Later in the afternoon, Trump told reporters at a Cabinet meeting that he hopes Romney will become a "team player."

Romney, who is set to take office Thursday, criticized the President's character in a Washington Post op-ed Tuesday, saying that Trump's "conduct over the past two years, particularly his actions this month, is evidence that the President has not risen to the mantle of the office."

"A president should demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect. ... And it is in this province where the incumbent's shortfall has been most glaring," Romney wrote.

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