Donald Trump has shrugged off a stinging personal attack on his presidency from senior fellow Republican Mitt Romney.
Mr Romney, who stood and lost for the Republicans against Barack Obama in 2012, suggested in an article that Mr Trump was not fit to be president.
Mr Trump had not "risen to the mantle", he wrote in the Washington Post.
At a White House cabinet meeting, the president said he hoped the incoming Utah Senator would be a team player.
Mr Trump said he was "surprised" Mr Romney criticised him so quickly, adding that "if he fought really hard against President Obama like he does against me, he would have won the election".
Earlier, the winner of the 2016 presidential election hit back by tweeting, "I won big and he didn't."
Mr Trump drew a comparison with Jeff Flake, an outgoing Republican senator who also attacked him last year.
But the president sounded conciliatory, calling for Mr Romney to be a "team player".
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
The timing of the article, just two days before Mr Romney is sworn in as senator for Utah, has prompted some to speculate he is positioning himself as a challenger to Mr Trump for the Republican presidential nomination.
What did the opinion piece say?
In the article, Mr Romney praised many of Mr Trump's policies, such as his tax reforms and appointment of conservative judges, and his crackdown on "China's unfair trade practices" - polices which he said "mainstream Republicans" had promoted for years.
But he went on to say: "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."
Policies and appointments are only a part of a presidency. A president must also unite us, inspire us, and defend our vital institutions. https://t.co/jmIw8XbV4k— Mitt Romney (@MittRomney) January 2, 2019
Mr Romney said he would support the president in policies he thought were in the best interests of Utah or the US but speak out against actions "that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions".
"The appointment of senior persons of lesser experience, the abandonment of allies who fight beside us, and the president's thoughtless claim that America has long been a 'sucker' in world affairs all defined his presidency down," he wrote.
"Trump's words and actions have caused dismay around the world."
The world "needs American leadership", he argued, and "the alternative... offered by China and Russia is autocratic, corrupt and brutal".
A fight for the future of Republicanism?
By Anthony Zurcher, BBC News, Washington
The on-again, off-again feud between Mitt Romney and Donald Trump is back on, although the president - for the moment - is shying away from open warfare.
That's a bit of a surprise, given then-candidate Trump unloaded with both barrels in 2016, when Mr Romney called him a fraud and a phony who should be opposed at every turn by Republican primary voters.
Perhaps the president realises he needs all the support he can get in the Senate, where another outspoken Republican, Jeff Flake, recently turned disapproving rhetoric into actions that stymied administration priorities.
Mr Trump would much prefer the new senator from Utah to follow the lead of other Republicans, who have paired the occasional criticism with lockstep support for the White House's agenda.
That may not be Mr Romney's game, however. Whether he is positioning himself to pick up the pieces after a possible Trump tumble or, perhaps, simply serve as a check on what he sees as the president's more dangerous impulses, the timing of this op-ed suggests this is an opening shot.
The two men in many ways represent the past and the present of Republicanism. Now they could be about to fight for the party's future.
How have other Republicans reacted?
Ronna McDaniel, chairwoman of the Republic National Committee and a niece of Mr Romney, described her uncle's article as "disappointing and unproductive".
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. https://t.co/ArhI7Bi7bo— Ronna McDaniel (@GOPChairwoman) January 2, 2019
Mr Trump's manager for the 2020 presidential campaign went so far as to accuse Mr Romney of jealousy.
The truth is @MittRomney lacked the ability to save this nation. @realDonaldTrump has saved it. Jealously is a drink best served warm and Romney just proved it. So sad, I wish everyone had the courage @realDonaldTrump had. https://t.co/mbxoTqbSX6— Brad Parscale (@parscale) January 2, 2019
Former White House press secretary Ari Fleischer said he was also "disappointed" in Mr Romney.
I’m disappointed in Mitt Romney. His defining act as an incoming Senator is to criticize Pres. Trump. If Senator-elect Romney thinks Trump is a bigger problem than Nancy Pelosi or Sen. Schumer then he has a lot to learn about how things get done in Washington.— Ari Fleischer (@AriFleischer) January 2, 2019
Meanwhile, the New York Times, the liberal daily regularly attacked by Mr Trump, commented that much of the essay "sounded like the makings of a primary challenge against Mr. Trump from Mr. Romney".
What kind of relationship do the two men have?
A complicated one. During the 2016 campaign, Mr Romney said Mr Trump had neither "the temperament nor the judgement to be president" while Mr Trump called Mr Romney a "choke artist" and called his bid for the presidency in 2012 "the worst ever".
The same year though, the pair met for dinner amid reports Mr Trump was mulling appointing Mr Romney as his secretary of state.
Mr Romney has also taken issue with Mr Trump labelling the press the "enemy of the people" as well as his response to a violent far-right rally in Charlottesville.
When the US Congress reconvenes on Thursday, Mr Trump faces a new challenge on the domestic front - in November's mid-terms the Republicans strengthened their hold of the Senate but lost the House of Representatives.
More women than ever before won seats in Congress in the 2018 mid-terms.