Donald Trump has hit back at Mitt Romney, labelling the Republican a "Flake" following fierce criticism over his policies and character.
The US president was accused of glaring shortfalls in character by Mr Romney, a former Republican contender for the White House.
Mr Trump responded by calling Mr Romney a "Flake" and suggested he should become a team player.
Mr Romney wrote an opinion piece in the Washington Post ahead of being sworn in as senator in Utah on Thursday.
In it, he said Mr Trump's conduct since being elected in 2016 "is evidence that the president has not risen to the mantle of the office".
He wrote: "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."
Mr Romney said all political parties could help restore America's leadership in world politics.
But he warned: "We must repair failings in our politics at home. That project begins, of course, with the highest office once again acting to inspire and unite us."
In response, Mr Trump tweeted: "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!"
Flake is a reference to Jeff Flake, a retiring senator who has been one of Mr Trump's critics within the party.
A passing reference to Brexit was also made, with Mr Romney calling for a "a unified and strong Europe, not a disintegrating union".
On how much he would support Mr Trump, the former Massachusetts governor vowed to act "as I would with any president, in or out of my party".
"I will support policies that I believe are in the best interest of the country and my state, and oppose those that are not," he explained.
"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."
Arizona senator Mr Flake tweeted with a link to the article: "A thoughtful piece by incoming Senator Romney."
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 didnt. He should be happy for all Republicans. Be a TEAM player & WIN!
— Donald J. Trump (@realDonaldTrump) January 2, 2019
Mr Trump's 2020 campaign manager Brad Parscale also said: "Jealousy is a drink best served warm and Romney just proved it."
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Mr Romney was a critic of Mr Trump during his election campaign in 2016, calling him "very, very not smart" and accusing him of "lacking the temperament to be president".
He also accused Mr Trump of mocking a disabled reporter, bragging about his marital affairs and letting "evil trump good".