Rob Portman, the Republican senator from Ohio, has announced he will not seek re-election next year, knocking out one of the party’s moderates as it tries to rebuild in the wake of Donald Trump’s presidential defeat.

Mr Portman, a former US trade representative and budget director under former president George W Bush, joins Pat Toomey of Pennsylvania and Richard Burr of North Carolina, other senators from his party who are bowing out at the end of their term.

In a statement on Monday, Mr Portman said it was a “tough time to be in public service”, criticising dysfunction in Congress with language that has become commonplace among retiring senators in recent years.

“Honestly, it has gotten harder and harder to break through the partisan gridlock and make progress on substantive policy, and that has contributed to my decision,” Mr Portman said.

“We live in an increasingly polarised country where members of both parties are being pushed further to the right and further to the left, and that means too few people who are actively looking to find common ground.”

Although Mr Portman hails from the Republican establishment, he did not go as far as some other GOP senators in criticising Mr Trump’s governing approach or efforts to overturn the results of the 2020 election. He has closely aligned with the former president on policy, particularly tax cuts and deregulation, over the past four years.

His retirement announcement will set off a big battle among Republicans to win their party’s primary for the seat next year — which could prove to be a litmus test for the strength of Mr Trump’s allies within the party compared with his critics.

Among the possible contenders is Jim Jordan, the Republican member of the House of Representatives, who has emerged as one of Mr Trump’s staunchest defenders on Capitol Hill. Mr Jordan voted to reject the results of the electoral college on the day of the deadly assault on Congress and was awarded the presidential medal of freedom by Mr Trump.

Mr Portman, 65, is relatively popular in Ohio, outperforming Mr Trump the last time he ran for office in 2016. His exit will make it easier for Democrats to attempt to pick up his seat in next year’s midterm election, when control of the upper chamber could hinge on the outcome of just a few races, given the current 50-50 split.

However, Ohio has been gradually tilting to the right, and was comfortably won by Mr Trump last November, so it will still be an uphill climb for Democrats to prevail.

A Cincinnati native, Mr Portman has a long history in Congress beginning with his tenure in the House of Representatives from 1993 to 2005, when he was tapped by Mr Bush to be US trade representative and later White House budget director.

Before Congress, Mr Portman had served as White House director of legislative affairs during the presidency of George HW Bush.