In a Facebook campaign video, he lamented that Washington was sending "a message of exclusion" to immigrants.

A vocal critic of Mr Trump, Mr Romney is seen as a shoo-in for outgoing Republican Senator Orrin Hatch's seat.

The ex-private equity manager has kept a low profile since losing his 2012 race to oust President Barack Obama.

Touting his management of the 2002 Salt Lake City Winter Olympics, Mr Romney said his adopted home state was a model for Washington DC.

"Utah welcomes legal immigrants from around the world," he said in a Facebook campaign video.

"Washington sends them a message of exclusion."

Since his failed White House campaign, Mr Romney has served as a chairman of his son's investment firm and on the board of directors of Marriott International.

 While he was in the running to be secretary of state, Mr Romney dined with Mr Trump at a Manhattan restaurant

But he has assiduously maintained his links to the Republican party.

In 2016 he positioned himself as the voice of mainstream conservatism with stinging attacks on Mr Trump.

"His promises are as worthless as a degree from Trump University," said Mr Romney.

Mr Trump hit back, calling Mr Romney a "failed candidate" who "choked like a dog" in the 2012 race.

Media captionMarch 2016 - Mitt Romney on Donald Trump: ''His promises are as worthless as a degree from Trump University''

However, once Mr Trump captured the White House, Mr Romney dined with him in November 2016 at a Manhattan restaurant while he was in the running to be the president-elect's secretary of state.

Mr Romney did not get the job.

Perhaps mindful of a Romney resurgence, the president recently urged Mr Hatch, seen as a Trump loyalist, to run again for another six-year term.

In December, Mr Trump travelled to Utah where he called Mr Hatch "a true fighter" and said he hoped he would continue to serve "in the Senate for a very long time to come".

Senator Hatch is the longest-serving Republican senator in US history

But the veteran senator announced last month he would step down at the end of the year.

"Every good fighter knows when to hang up the gloves," said Mr Hatch.

The longest-serving Senate Republican with seven terms under his belt, he was first elected in 1976.