Mitt Romney, the former Republican presidential candidate and incoming US senator for Utah, has sharply criticised Donald Trump both personally and professionally, accusing him of deserting the nation’s allies and lacking the character to lead a “divided” nation.

Romney said on Wednesday afternoon that it was Trump’s abrupt announcement last month that he intended to pull US troops out of Syria immediately that prompted the incoming senator to write an opinion article criticizing the president.

In a Washington Post essay published on Tuesday evening, Romney said Trump’s presidency “made a deep descent” in December and blamed him for the departures of the defence secretary, Jim Mattis, and White House chief of staff, John Kelly.

“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.

Will Mitt Romney be a thorn in Trump's side if he wins Utah Senate race? Read more

“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.”

Trump responded to Romney’s criticism on Wednesday morning on Twitter, calling on him to be a team player.

Donald J. Trump (@realDonaldTrump) 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!

Romney then said in a TV interview with CNN on Wednesday afternoon that he was prepared to work with any president – but was willing to speak out when he did not agree with them.

“It’s important, as I begin this new job, to make it very clear where I stand. And I also note that the departure of Secretary [James] Mattis and the decision to pull out of Syria and the abrupt way it was done was a precipitating event for my finally going on this record,” he said.

Romney praised Trump’s tax policies, stance on China and appointment of conservative judges but said they were “mainstream” Republican policies. Since Trump’s rise, Romney has been one in a long list of traditionalist Republicans who have publicly bristled at Trump’s decorum and leadership style, while generally supporting his policy goals.

“To a great degree, a presidency shapes the public character of the nation,” he wrote. “A president should unite us and inspire us to follow ‘our better angels’. A president should demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect. As a nation, we have been blessed with presidents who have called on the greatness of the American spirit.

“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.”

He added that “Trump’s words and actions have caused dismay around the world” and cited Pew reports suggesting belief among people in Germany, Britain, France, Canada and Sweden that the US president would “do the right thing” had fallen from 84% in 2016, when Barack Obama was president, to 16% a year later under Trump.

The White House did not immediately respond to a request for comment. Trump’s 2020 campaign manager, Brad Parscale, tweeted late on Tuesday to say that Romney “lacked the ability to save this nation”, adding: “Jealousy is a drink best served warm and Romney just proved it.”

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Romney is staking out an independent position two days before he takes office in the GOP-controlled Senate on Thursday. It is unclear whether Trump will face a serious challenge in 2020 to securing the Republican party’s presidential nomination. It would be the first time since 1992 that an incumbent had faced such a challenge.

Trump remains extraordinarily popular with Republican voters, boasting an approval rating that hovered near the low 90s through the end of the year. But, as the Mueller investigation winds on into its third calendar year, Trump also faces increased scrutiny, with an incoming Democratic House preparing to launch a number of additional inquiries into Trump’s conduct and presidency.

Romney, who has run for president twice, has long been viewed as a potential challenger to Trump’s 2020 ambitions, and the two men have had a complicated political relationship.

Trump endorsed Romney’s run for a Senate seat in Utah in February last year, as well as Romney’s 2012 presidential campaign.

But during the 2016 presidential campaign, Romney excoriated Trump as a “fraud” who was “playing the American public for suckers”. Trump responded that Romney had “choked like a dog” in his unsuccessful 2012 campaign against Barack Obama.

Despite Romney’s prior criticism, after Trump won the presidency in November 2016, Trump briefly considered tapping Romney as secretary of state.

In his essay on Tuesday, Romney said he “will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions”.

Reuters contributed to this report

• This article was amended on 3 January 2019. The findings of the Pew reports cited by Mitt Romney related to confidence in who was US president at the time of the research to “do the right thing”, rather than specifically confidence in Donald Trump as an earlier version said.