Washington, Jan 3 (TIWN / IANS) Former Republican presidential candidate and incoming Utah Senator Mitt Romney has launched a scathing attack on President Donald Trump, writing in an editorial that he caused dismay around the world and had "not risen to the mantle of the office".

"It is well known that Donald Trump was not my choice for the Republican presidential nomination. After he became the nominee, I hoped his campaign would refrain from resentment and name-calling. It did not," Romney wrote in the Washington Post opinion piece on Tuesday.

"... His conduct over the past two years, particularly his actions, is evidence that the President has not risen to the mantle of the office."

Romney, who is set to be sworn into the Senate on Thursday, also said that Trump's presidency weakened America's influence abroad, writing that "Trump's words and actions have caused dismay around the world".

"The world needs American leadership and it is in America's interest to provide it."

"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."

Trump hit back at the Republican senator-elect in a tweet on Wednesday, saying: "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 Senator Jeff Flake, a Republican who 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 and win!" Trump said.

Romney said that he agreed with some of the policy changes Trump has championed. "But policies and appointments are only a part of a presidency," he wrote.

Responding to Romney's piece, Trump's 2020 campaign manager Brad Parscale said that the incoming senator "lacked the ability to save this nation" while Trump "has saved it".

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

Romney and Trump have long had a complicated political relationship. The President had called Romney "irrelevant" and had bragged that he was a more successful businessman. 

When Trump was running for President in 2016, Romney called him a "phoney" and a "fraud". In 2017, he slammed the President after the deadly white supremacist rally in Virginia for causing "racists to rejoice" and "minorities to weep".

Romney said he would support the President in policies he thought were in the best interests of Utah or the US but speak out against actions "that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions".