Prominent Republican Mitt Romney has launched a scathing attack on US President Donald Trump, only days before taking a seat in the US Senate.

Mr Romney - who was the Republican presidential candidate against Barack Obama in the 2012 presidential election - is due to be sworn in as a Republican senator for Utah tomorrow.

In an op-ed for the Washington Post published overnight, Mr Romney claims Trump has "not risen to the mantle of the office".

He claims that the President has been "right" in his adherence to some long-standing Republican policies, but adds that the US leader's words and actions have also "caused dismay around the world".

Mr Romney writes: "I will support policies that I believe are in the best interest of the country and my state, and oppose those that are not. 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 adds: "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."

The comments prompted a swift Twitter response from President Trump:

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!

— Donald J. Trump (@realDonaldTrump) January 2, 2019

"A phony"

Mr Romney was one of the most high-profile Republican critics of Donald Trump during the 2016 presidential campaign, claiming: "Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University."

He was later, however, photographed dining with then president-elect Trump, reportedly as one of those under consideration for the Secretary of State role.

Romney had previously unsuccessfully ran for a Senate seat in 1994, but rose to political prominence as Governor of Massachusetts.

Launching his ultimately successful Senate bid last year, the veteran politician claimed he wanted to "bring Utah's values to Washington".

This week will see significant changes in the US Congress: while Republicans will slightly increase their existing majority in the Senate, Democrats will take over the House of Representatives.

It'll be the first time since the 2016 election that Republicans will not control the three main legislative bodies in Washington.

Despite the changes, politicians from both parties will quickly find themselves tasked with attempting to resolve the ongoing partial government shutdown.

Hundreds of thousands of federal workers have been sent home or required to work without pay for almost two weeks, amid a political impasse caused by the President's demands for funding for a border wall with Mexico.