WASHINGTON – It looks like President Donald Trump has a new Republican antagonist in the Senate.

His name is Mitt Romney.

Responding to a critical op-ed by the Utah Republican, Trump tweeted Wednesday that the incoming Utah senator – and 2012 Republican presidential candidate – should be more respectful.

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

In an op-ed published by the Washington Post, Romney said Trump's leadership is unnecessarily dividing Americans and alienating allies. He questioned Trump's character, and said he has not "risen to the mantle of the office."

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

Romney welcomed Trump's support for his own presidential bid in 2012. He later criticized Trump when the New York businessman successfully sought the Republican presidential nomination in 2016.

After winning the 2016 election, Trump interviewed Romney for secretary of State, though it's hard to say how serious that courtship was.

Now elected to the Senate from Utah, Romney may seek to become a prominent Republican critic of Trump, a role that had been filled by the late Sen. John McCain and outgoing Sen. Jeff Flake, both of Arizona, and outgoing Sen. Bob Corker of Tennessee.

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

More:Mitt Romney on President Donald Trump: He hasn't 'risen to the mantle of the office'

Romney was also attacked by a prominent Trump backer: Republican Party chairwoman Ronna McDaniel – who also happens to be Romney's niece.

"POTUS is attacked and obstructed by the MSM media and Democrats 24/7," McDaniel tweeted. "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."