Utah Senator Orrin Hatch announced he plans to retire, opening the door for prominent Donald Trump critic Mitt Romney to run.

The departure of Mr Hatch will almost certainly not alter the political balance of the Senate, where Republicans hold a slender 51-49 majority. A reliably conservative state, Utah is likely to elevate another Republican to take Mr Hatch’s place.

But should Mr Romney choose to run and win, the Senate would effectively swap out a key ally of Mr Trump for a vocal detractor. Mr Hatch said he plans to step down when his current term expires, adding his seat to the column of those in play during the 2018 election.

A former Massachusetts governor and Republican presidential nominee, Mr Romney has established residency in Utah. He is perhaps America’s best-known Mormon public figure, which would likely buoy his prospects in the religion’s heartland.

While he has made no formal declaration of his intentions, Mr Romney has reportedly discussed with associates the possibility of running should a seat open up. Speculation about his plans immediately followed the news of Mr Hatch’s departure.

Some Senate Republicans have openly hostile relationships with Mr Trump, who shows few compunctions about publicly attacking members of the caucus. But Mr Hatch has been a reliable ally, and the admiration has been mutual.

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“We hope you will continue to serve your state and your country in the Senate for a very long time to come,” Mr Trump said of Mr Hatch during a December speech marking his decision — lauded by Mr Hatch — to shrink a pair of national wildlife areas in Utah.

But Mr Hatch's has decision to not do so despite the President's urging offers an opportunity to Mr Romney. Both during the 2016 presidential campaign and Mr Trump’s tenure, the former Republican presidential standard-bearer has been among the more vocal Republican detractors of a President who continues to clash with establishment party figures.

In March of 2016, before Mr Trump had secured the nomination, Mr Romney assailed Mr Trump as a “fraud”.

“His promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers. He gets a free ride to the White House, and all we get is a lousy hat,” Mr Romney told a University of Utah audience.

Mitt Romney takes down Trump

Mr Trump returned fire, repeatedly mocking Mr Romney’s “failed” candidacy on Twitter.

After briefly exploring a White House role, which resulted in a notorious photo of an uneasy-looking Mr Romney dining with the then-President-elect, Mr Romney has continued to offer a voice to Republicans who remain opposed to Mr Trump’s presidency.

When Mr Trump offered an equivocal response to a white supremacist driving into a crowd of counter protesters in Virginia, saying there was blame on “both sides,” Mr Romney released a long statement demanding that the President condemn hate groups like neo-Nazis (the President did, before returning to spreading blame).

He has also embraced the Paris Climate agreement from which Mr Trump withdrew, denounced failed Alabama Senate candidate Roy Moore, whom Mr Trump backed, and lauded as “Lincolnesque” a speech from Sen John McCain that rejected “half-baked, spurious nationalism” in a thinly veiled dig at Mr Trump.

Mr Romney posted a Facebook statement thanking Mr Hatch for his service, and the divergent responses helped illuminate the political climate into which Mr Romney might tread.