Sen.-elect Mitt Romney Willard (Mitt) Mitt RomneyThe Memo: Warning signs flash for Trump on debates GOP votes to authorize subpoenas, depositions in Obama-era probe Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE (R-Utah) slammed President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE's character on Tuesday, writing in a Washington Post op-ed that it fell short of what the country needed.

"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," Romney wrote.

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Romney, who criticized Trump in the run up to the 2016 election, said the events of the past month, including the departures of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and chief of staff John Kelly, marked a worrying decline for Trump's presidency.

"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," the 2012 Republican presidential nominee wrote.

Romney argued that Trump's "shortfall" set a poor example for America's "public character," while alienating U.S. allies.

"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," he wrote. "To reassume our leadership in world politics, 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," he went on.

Romney added that not all of Trump's policies were "misguided," but said the president should "unite us and inspire us to follow 'our better angels.'"

"To a great degree, a presidency shapes the public character of the nation. A president should unite us and inspire us to follow 'our better angels,'" he wrote.

Romney's comments come two days before he will be sworn in to the Senate on Thursday. With the op-ed, Romney seems poised to become one of Trump's top Republican critics in the Senate.

Romney's swearing in Thursday will follow the departure of some of the Senate's most vocal Trump critics, such as Sens. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) and Jeff Flake Jeffrey (Jeff) Lane FlakeRepublican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style Bush endorsing Biden? Don't hold your breath MORE (R-Ariz.).