"Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not," Trump tweeted, referring to retiring Arizona Sen. Jeff Flake, a Republican who has criticized the President often in the last few years. "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!"
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Later in the afternoon, Trump told reporters at a Cabinet meeting that he hopes Romney will become a "team player."
Romney, who is set to take office Thursday, criticized the President's character in a Washington Post op-ed Tuesday, saying that Trump's "conduct over the past two years, particularly his actions this month, is evidence that the President has not risen to the mantle of the office."
"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," Romney wrote.
The incoming Utah senator also said that he does not "intend to comment on every tweet or fault," but that he will "speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions."
But by-and-large the President held his fire Wednesday afternoon on the onetime presidential nominee, choosing not to issue any overt insults or criticism of a new congressional foil when speaking to reporters during his cabinet meeting.
"I wish Mitt could be more of a team player. I'm surprised he did it this quickly. I was expecting something, but I'm surprised he did it this quickly," Trump said during a meeting of his Cabinet.
Trump noted he'd endorsed Romney in his election, a nod for which Trump said Romney "thanked me profusely."
"I was happy that he won in Utah," Trump said. "I have great popularity in Utah."
Trump did suggest the new senator could have won his own presidential bid if he'd been tougher.
"I will say this, if he fought really hard against President Obama like he does against me, he would have won the election," Trump said. "If he fought the way he fights me, I'm telling you, he would have won the election.
Romney was seen as a "Never Trumper" during the 2016 election.
Now with Flake on the way out, Romney could take up the role of being an outspoken Republican critic of the President in Congress.
In his speech announcing his retirement, Flake denounced the "complicity" of his own party in what he called an "alarming and dangerous state of affairs" under Trump and blamed the President for setting the tone. Flake pointed to Romney's op-ed on Tuesday as "thoughtful."
Republican National Committee Chairwoman Ronna McDaniel, who is Romney's niece, slammed her uncle in a tweet about the op-ed, only referring to Romney as a "freshman senator."
"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," McDaniel tweeted.
Romney's relationship with Trump has been complicated over the years. When Romney sought the presidency in 2012, Trump endorsed the former Massachusetts governor, calling him "tough" and "smart."
But during the 2016 election, Romney delivered a scathing rebuke of Trump in a speech in which he called the Republican candidate a "phony" and a bully, and criticized his rhetoric about Muslims and Mexican immigrants.
Trump retaliated by mocking Romney's 2012 presidential loss and claimed Romney was "begging" for his endorsement during that time.
After Trump was elected, the two men seemed to mend fences and Romney was briefly considered for the position of secretary of state.
When he announced his run for senator, Romney appeared to take a veiled swipe at the Trump administration's immigration policies. But his criticism of Trump was far more muted and in February, Trump endorsed Romney.
Romney will be sworn in Thursday along with the rest of the incoming Senate by Vice President Mike Pence.