WASHINGTON (BLOOMBERG) - Senator Orrin Hatch of Utah, the longest-serving Republican senator in US history and a leader of the effort to rewrite the US tax code at the end of last year, said he won't seek an eighth term in 2018.

Hatch, 83, is a steadfast conservative who is chairman of the Senate Finance Committee, which oversees tax policy.

"Every good fighter knows when to hang up the gloves, and for me that time is soon approaching," Hatch said in a video posted on Twitter.

"I've decided to retire at the end of this term."

The decision may open the way to a Senate bid by Mitt Romney, the former Massachusetts governor and two-time GOP presidential contender. Romney, who was passed over by President Donald Trump for secretary of state, now lives in Utah and has been making the rounds at political events in the state, but hasn't publicly said whether he will run.

Romney, 70, is viewed as a Republican elder statesman who could challenge Trump, whom he has called a "fraud."

First elected in 1976, Hatch is now the presiding officer of the Senate, a position third in the presidential line of succession.

His four decades in the Senate have spanned seven presidencies and included his own unsuccessful Oval Office bid in 2000, which ended when he finished last in the Iowa caucuses. He's been chairman of three Senate committees: Finance, Judiciary, and Health.

Hatch has been a hard-line backer of conservative causes such as tax cuts, gun rights, opposition to abortion, and a balanced-budget constitutional amendment.

While Hatch has a genial demeanour, he can be a fierce partisan. In November he strongly criticised Democrats on his committee who opposed the GOP tax-cut plan championed by party leaders and Trump.

In 2009, he was the first Republican to quit a seven-member bipartisan Senate Finance Committee group that sought to craft what eventually became Obamacare. He told reporters he doubted it was something he could ever support after learning what Democrats were seeking in a health-care overhaul.

Yet, at the same time, he has a serious track record of bipartisanship. He joined with the late Senator Ted Kennedy, a Massachusetts Democrat, in sponsoring the Americans with Disabilities Act and legislation creating the Children's Health Insurance Programme, which extends health coverage to low-income kids. With Democratic Senator Patrick Leahy of Vermont, he has pushed through key changes to patent law.

And in 2013, Hatch was one of 14 Senate Republicans who voted for immigration legislation that would have provided a path to legal permanent residency for some undocumented immigrants.

Hatch's willingness to reach across the aisle made him a top target of the small-government Tea Party movement in 2012, two years after it derailed Utah Republican Senator Robert Bennett's bid for a fourth term.

Hatch spent months wooing Utah Tea Party activists, and while he failed to get enough support from state GOP convention delegates to avoid a primary, he defeated his primary opponent and went on to win a seventh term.

Hatch aided Trump in the 2016 campaign when the bombastic New Yorker got a tepid reception from Utah's socially conservative Mormons. Hatch, himself a Mormon, told state delegates at the Republican National Convention they should look past their concerns and focus on the potential to appoint Republican-leaning justices to the Supreme Court.

After Trump's victory, Hatch helped lead the unsuccessful effort to replace the Affordable Care Act and played a key role in the drive to rewrite the tax code and cut the corporate tax rate. As a senior member of the Senate Judiciary Committee, he also helped champion the confirmation of Supreme Court Justice Neil Gorsuch, Trump's pick to replace the late Justice Antonin Scalia.

Republicans are heavily favoured to win Hatch's seat. Utah gave 45.5 per cent of its vote to Trump in 2016, while Democratic presidential nominee Hillary Clinton received just 27.5 per cent and independent candidate Evan McMullin got 21.5 per cent.

The 2018 Utah Senate race has not yet taken shape. Democratic Salt Lake County Councilwoman Jenny Wilson is running, and independent McMullin is another potential candidate.

Romney has long been a Trump critic. During the 2016 campaign, Romney emerged as a leader of the GOP "Never Trump" movement, attacking the billionaire business owner as a "phony" and urging primary voters to pick someone else.

Tensions between the two seemed to relax after the election, when Trump considered Romney for secretary of state, but he chose Rex Tillerson instead.