Romney, a vocal critic of Trump, wrote that the president's "conduct over the past two years, particularly his actions this month" indicate that he 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", Romney said.
"A president should unite us and inspire us to follow 'our better angels, '" the failed 2012 candidate continued.
Romney said he will treat Trump as he would any president, either in or outside his party.
The world "needs American leadership", he argued, and "the alternative. offered by China and Russian Federation is autocratic, corrupt and brutal".
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 news: India beats Australia in Boxing Day Test to retain Border-Gavaskar Trophy
Some analysts have speculated that Romney's Senate seat could set him up for another presidential run, possibly as soon as 2020, when Trump will seek re-election.
The newly-minted US senator from Utah then lamented that President Trump stands out of line with the great presidents of the past. In a tweet Tuesday night, Parscale said Romney "lacked the ability to save this nation" and contended that Trump "has saved it". So, instead of coming to Washington as a problem solver like he campaigned, former Massachusetts Governor Mitt Romney chose to start his tenure as Utah's junior United States Senator as part of the problem.
Democratic Sen. Dick Durbin, D-Ill., said he suspects the 2020 elections will prompt more Republican senators to confront Trump when they disagree with him, and that may already be happening, citing recent actions on Saudi Arabia. There is no point to this op-ed.
During the 2016 presidential campaign, Romney excoriated Trump as a "fraud" who was "playing the American public for suckers". (Romney did, however, get a larger percentage of the national popular vote in 2012, at 47.2 percent, than Trump did in 2016, with 46.1 percent.) Paul wrote off Romney in general as standing for a sort of "establishment big government Republicanism" that is "not popular enough to win a general election".
Trump responded to Romney on Wednesday in a tweet and during a cabinet meeting in the White House. "I hope not", Trump said.
"He should be happy for all Republicans", Trump continued of Romney. He also praised his colleagues who keep their disagreements with the president on policies like tariffs in private, rather than publicly attacking the president.
"POTUS is attacked and obstructed by the [mainstream] media and Democrats 24/7", she said. The op-ed rocked the Republican party, and triggered speculation that Romney could assume the role of Trump skeptic in a Senate that's losing its more independent GOP voices.