Republican Sen. Orrin Hatch, the longest-serving conservative in the US upper house of Congress, on Tuesday announced his retirement at age 83, saying that he will not run for re-election in 2018.

Hatch, who represents the state of Utah, thus ignored the pleas of President Donald Trump, who has been urging him to stand for re-election to his Senate seat, which he has held for the past 40 years.

The respected member of the GOP is the president pro tempore of the upper house, a position that makes him third in the presidential line of succession after Vice President Mike Pence and House Speaker Paul Ryan.

During his long Senate career, Hatch has earned a reputation for bipartisanship, having sponsored legislation with the late Democratic Sen. Ted Kennedy and other Democrats, as well as supporting most of former President Bill Clinton's judicial nominees. He is currently chairman of the Senate Finance Committee.

Hatch's withdrawal from the re-election race opens the door for another key GOP figure - 2012 presidential nominee Mitt Romney, who has become one of the biggest critics of Trump's presidency - to aspire to win the Senate seat vacated by the Utahn, according to local media.

In a video posted on the Internet, Hatch said that he has always been a fighter and he had tried to bring that spirit to the public institutions, but he added that "every good fighter knows when to hang up the gloves."

In the video statement, the conservative said that he made his decision not to seek re-election and retire at the end of this legislative session after consultation with his family over the Christmas holiday.

Meanwhile, White House Press Secretary Sarah Huckabee Sanders said at a press conference that Trump is "very sad" over Hatch's departure, adding that the president has the greatest respect for him and emphasizing his efforts to ensure passage of the recently approved tax reform bill.

Trump had said at a political rally in Utah that he hoped Hatch would keep his Senate seat and continue serving the country in a public capacity.