Days away from joining the Senate's Republican majority, senator-elect Mitt Romney broadly criticized U.S. President Donald Trump's policies and character and said that the president "has not risen to the mantle of the office."

"With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable," the Utah Republican and 2012 presidential nominee wrote in a Washington Post op-ed posted online Tuesday night. "And it is in this province where the incumbent's shortfall has been most glaring."

Romney's biting public assessment came as Trump and Senate Republicans faced a new governing dynamic. Republicans on Thursday will cede control of the House to Democrats, who seem prepared to oppose Trump on a number of policies and promised a slew of investigations into his actions and those of his aides and campaign officials, particularly with regard to Russia's election meddling.

Trump responded to the criticism in a tweet early Wednesday.

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! —@realDonaldTrump

Trump's warning shot referred to retiring Sen. Jeff Flake of Arizona, who has been Trump's most consistent critic among Senate Republicans. As he and other critics leave Congress, it is an open question who — if anyone — will take up the role of publicly criticizing a president who remains popular with Republican voters.

Romney, a former Massachusetts governor, had criticized Trump before — notably, in a March 2016 speech he called Trump a "fraud" and opposed his bid for the Republican nomination — but later he made peace with the president-elect and even expressed interest in joining his administration.

He has also accepted Trump's endorsements twice — during his unsuccessful 2012 presidential campaign, and during his Senate race last year.

Trump greets Romney, then a Republican presidential candidate, after announcing his endorsement of Romney in Las Vegas in February 2012. (Julie Jacobson/Associated Press)

Trump said during a cabinet meeting Wednesday that he was surprised by Romney's comments. "People are very upset with what he did," Trump said. He also referenced Romney's 2012 election loss to former president Barack Obama.

"If he fought the way he fights me, I'm telling you, he would have won the election," Trump said.

Romney later told CNN that there "are places where we agree on a whole series of policy fronts, but there are places that I think the president can, if you will, elevate his game and do a better job to help bring us together as a nation."

Asked if he would endorse Trump for president in 2020, Romney said, "I'm going to wait and see what the alternatives are."

Romney ruled out another run himself: "You may have heard, I ran before," he said.

Romney's rebuke of Trump drew a cutting reply from Brad Parscale, Trump's campaign manager. In a tweet Tuesday night, Parscale said Romney "lacked the ability to save this nation" and contended that Trump "has saved it."

"Jealously is a drink best served warm and Romney just proved it," Parscale wrote. "So sad, I wish everyone had the courage [Trump] had."

In a conference call with reporters, Republican Sen. Rand Paul of Kentucky noted that the 2020 election hopes of many Republicans in the Senate and House will be tied to Trump. Paul said he disagrees with the president on some issues, but still treats him with respect.

"I don't think the president deserves a new senator coming in attacking his character," Paul said.

Republican National Committee chair Ronna McDaniel also criticized Romney on Twitter, calling his essay an "attack" on Trump that was "disappointing and unproductive." McDaniel is Romney's niece.

'Glaring' shortfall

In the column, Romney offered approval of Trump's corporate tax policies and efforts to cut regulations, appoint conservative judges and other "policies mainstream Republicans have promoted for years. But policies and appointments are only a part of a presidency."

"To a great degree, a presidency shapes the public character of the nation," Romney said. He later added: "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."

While saying Trump's early administration appointments had been encouraging, Romney added that, "on balance, his 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."

In describing a "deep descent" by the Trump presidency in December, Romney cited the departures of Defence Secretary Jim Mattis and White House Chief of Staff John Kelly and what he called the appointment of people of lesser experience, the abandonment of allies, and Trump's "thoughtless claim that America has long been a 'sucker' in world affairs."

Looking ahead, Romney wrote that he would act as he would with any president from either party in the White House, supporting policies he believes are in the best interest of the country and his state, and opposing those that are not. He said he didn't 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," he said.

Romney will be sworn in as a senator on Thursday.