The new session of Congress begins on Thursday, January 3, but even before that, newly elected Sen. Mitt Romney (R-Utah) fired off some blistering criticism of President Donald Trump in a new opinion piece this week, claiming Trump “has not risen to the mantle of the [presidential] office.”

Romney will be sworn in Thursday with the rest of the new Congress.

He wrote in an op ed published in The Washington Post on Tuesday that he will not respond to every tweet Trump writes — and that he will back policies that are in the interest of the United States, but that he will “oppose those that are not.”

“I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions,” Romney also said.

He also wrote, “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.”

The commander-in-chief responded to the comments on Wednesday morning, saying Romney should be a “team player and 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.”

He added, “I won big, and he didn’t. He should be happy for all Republicans. Be a TEAM player & WIN!” Trump wrote on Wednesday.

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Trump was referring to the now-retired Sen. Jeff Flake (R-Ariz.), who was a ceaseless Trump critic over the past year.

A Twitter commenter strongly agreed with Trump on Wednesday morning, writing, “Agree. He [Romney] should be a team player. He lost. Stay strong, Mr. President. You have many haters, but we the majority of the American people support you. Stay strong, sir!”

Romney has a habit of criticizing Republican presidents, as Reagan historian and regular LifeZette contributor Craig Shirley pointed out Wednesday morning in a tweet:

Not only have Trump and others rebuked Romney for his comments, but Romney’s own niece took umbrage at the remarks.

Ronna McDaniel, the Republican National Committee chairwoman also known as Ronna Romney McDaniel, scolded her uncle on Twitter.

“POTUS is attacked and obstructed by the MSM media and Democrats 24/7. For an incoming Republican freshman senator to attack @realdonaldtrump as their first act feeds into what the Democrats and media want and is disappointing and unproductive,” she tweeted.

On Wednesday morning, Sen.-Elect Marsha Blackburn (R-Tenn.) commented on the issue on Fox News.

“There is an acronym for the word ‘team’ and it’s something my mom used with me when we were growing up — ‘together everyone achieves more.’ And what Tennesseans are looking at is a president who has delivered on turning the economy around. Promise kept!” she said.

“He has delivered on moving the [U.S.] embassy [in Israel] to Jerusalem … [Trump] has delivered when it comes to defeating ISIS, [on] dealing with China, dealing with trade, and getting us back on the road to economic prosperity. Tennesseans want to see more of that action, and I’ll tell you something else they want to see — and that’s border security,” Blackburn added.

She said her constituents are looking for some very specific things from her in the years ahead: “‘Do everything that you can possibly do to help keep this nation free and keep it prosperous.'”

She stressed the five broad areas of focus she will have beginning this week as the new Congress begins its work: “Faith, family, freedom, hope and opportunity” for America.

In an interview with CNN after his opinion piece was posted, Romney said he criticized Trump because of the president’s decision to pull U.S. troops out of Syria “and the abrupt way it was done.”

When it was pointed out to him that he had accepted Trump’s endorsement of him for his Senate run, Romney replied, “He was endorsing me. I wasn’t endorsing him.”

Romney was asked if he was considering a 2020 run against Trump. “No,” he said.

He also acknowledged that Trump “was successful” in his presidential bid, and “I was not. He did something I couldn’t do. He won … But no, I’m not running again.”

Check out this video of Marsha Blackburn’s comments on Wednesday morning:

This article has been updated.