US Senator Mitt Romney will not seek reelection in 2024, capping a roller-coaster ride through Republican politics from the height of his party's 2012 presidential nomination to the depths of tribal warfare in the age of Donald Trump.

Casting aside the hopes and appeals of colleagues including Senate Republican Leader Mitch McConnell, the 76-year-old Utah Republican said he would retire as a one-term senator when his term ends in early 2025, rather than seek another six years among a dwindling number of Republican moderates in Congress.

"At the end of another term I'd be in my mid-80s. Frankly it's time for a new generation of leaders," Romney said in a video statement released on Wednesday. "While I'm not running for reelection, I'm not retiring from the fight."

The son of a former Michigan governor and auto industry executive, Romney became a multimillionare in the private equity business and served as governor of Massachusetts before mounting an unsuccessful challenge against former President Barack Obama as the Republican party presidential nominee in 2012.

As a US senator since 2019, he has been an outspoken critic of Democratic President Joe Biden, though one willing to work with the White House and Democrats on issues including infrastructure and gun control.

But within Republican circles, Romney stood out as a rare Trump critic who voted to convict the former president at two Senate impeachment trials.

With Trump dominating the 2024 Republican presidential field, Romney has faced powerful headwinds at home in solidly Republican Utah. A Deseret News poll in June showed 47 percent of Republicans saying that Trump best represented them, compared with 39 percent who favoured Romney.

Trump called Romney's retirement "fantastic news for America" in a social media post on Wednesday. (Reuters)