WASHINGTON (CBS) – President Donald Trump has responded to a critical op-ed from former Massachusetts Gov. Mitt Romney, who is set to be a sworn in as a U.S. Senator from Utah this week.

Romney, the 2012 Republican party presidential nominee who spoke out against Trump during the 2016 election and at times during his presidency, wrote a piece in The Washington Post arguing that “the president has not risen to the mantle of the office.”

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

Trump hit back Wednesday morning.

“Here we go with Mitt Romney, but so fast! Question will be, is he a Flake?” Trump tweeted, referencing outgoing Arizona Sen. Jeff Flake, a frequent critic.

Trump called on Romney to focus on “Border Security” and to be a “TEAM player.”

The president’s 2020 campaign manager Brad Parscale responded to Romney Monday night, tweeting that he “lacked the ability to save this nation,” and “jealously is a drink best served warm.”

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Romney wrote in his op-ed that he doesn’t plan to comment every time he disagrees with Trump, but intends to speak out when the president is being “divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.”