Orrin Hatch, Republican Senator since 1977, won't run this year, opening path for Romney

Republican Sen. Orrin Hatch of Utah said Tuesday he will not seek re-election after serving more than 40 years in the U.S. Senate.

Hatch's retirement opens the door for Republican Mitt Romney to run for his seat, as has been rumoured in recent weeks.

Hatch, 83, says he's always been a fighter, "but every good fighter knows when to hang up the gloves."

He is the longest-serving Republican in the Senate and chairs the powerful Senate Finance Committee, making him a major force in getting a tax overhaul approved in December.

Hatch said he decided to retire at the end of his seventh term having spanned the terms of seven U.S. presidents after "much prayer and discussion with family and friends" over the holiday break.

In a video message posted on social media, Hatch thanked the voters and evoked his upbringing in Pennsylvania during the Depression.

"Only in a nation like ours could someone like me, the scrappy son of a simple carpenter, grow up to be a U.S. Senator," he said.

"I may be leaving the Senate, but the next chapter in my public service is just beginning," Hatch added, without elaborating.

Romney speculation likely

White House Press Secretary Sarah Huckabee Sanders said Hatch's leadership will be greatly missed.

Huckabee Sanders said President Donald Trump "has the greatest and deepest amount of respect for Senator Hatch and his over four decades of experience in the Senate."

Hatch also served as chairman of the Senate Judiciary Committee and was at the centre of many of the biggest confirmation battles.

In 2000, Hatch dabbled in presidential politics himself. He sought the Republican nomination for president, saying he had more experience in Washington than his opponents and that he could work with Democrats. Hatch readily acknowledged that winning would be a long shot.

He withdrew from the race after winning only one per cent of the vote in the Iowa caucuses. He then endorsed George W. Bush.

Prior to entering politics he served as an attorney in Utah, having attended the state's biggest school, Brigham Young University.

Romney maintains residence in Utah and was the president and CEO of the Salt Lake City Olympic Committee before going on to serve as governor of Massachusetts and challenge Barack Obama as the Republican nominee in the 2012 presidential election.

Despite his high profile, Romney has never served in a political capacity in D.C.

Romney wasn't tipping his hand on social media on Tuesday, preferring to praise Hatch for his long service.

Huckabee Sanders said it is premature to speculate as to whether Trump would campaign for the Republican candidate who will emerge in Utah.

She said she wouldn't "weigh in" on whether Trump would be comfortable with Romney as a candidate, given that the pair have previously lobbed public criticism at each other.

Utah is one of 32 Senate races in the 2018 midterm elections and one of just eight seats that the Republicans will have to defend. Of those eight seats, at least three will feature new Republican faces following announcements that Jeff Flake (Arizona) and Bob Corker (Tennessee) won't run for election again.

Unlike Flake and Corker, Hatch hasn't clashed with Trump.

He played a key role in persuading the president to sign proclamations scaling back two sprawling national monuments in Utah that Hatch and other conservatives considered example of government overreach.

The controversial move will likely be tied up in the courts for some time, with Indigenous tribes and environmentalists slamming it as being beholden to energy companies who see the lands as resource-rich.

In November, Hatch called Trump "one of the best" presidents he's served under.