The Californian congressman must win a majority vote in the House on Tuesday to get the role.

But a group of Republicans is refusing to back him, despite him making a number of last-minute concessions.

McCarthy cannot afford to lose more than a handful of votes from his party if he is to win the vote.

The role of Speaker of the House is one of the most important jobs in US politics.

They control the legislative agenda and timetable in the House, as well as who sits on various committees.

Failure to win the speakership on the first attempt could therefore weaken both McCarthy and the Republicans' credibility, hampering House Republicans' plans of acting quickly to investigate the Biden administration as well as President Joe Biden's son, Hunter Biden.

In November's mid-term elections, the Republican Party narrowly secured control of the House of Representatives (the lower chamber of the US Congress) from the Democrats, winning 222 of the 435 seats.

Following the result, the Republican Party's leader, McCarthy, ran to be the party's candidate for the new Speaker and won with 188 votes.

Since then, he has been working to gain the support of Republicans ahead of the opening of the new Congress on Tuesday, when the vote will take place. 35 new US senators will also be sworn into the Senate.

If Mr McCarthy wins, he will replace Democrat Nancy Pelosi as Speaker of the House.

But some right-wing Republicans are refusing to support him, making his bid for the speakership less certain.

The narrowness of the Republicans' majority in the House means that McCarthy can only afford to lose four of their votes if he is to secure a majority of 218 and secure the position of speaker.

Those who oppose McCarthy's bid for the speakership are Trump-supporting hardliners, dubbed the "Never Kevins" by some.

They include representatives Bob Good and Matt Gaetz, who believe McCarthy represents too much of the mainstream and the establishment.

"I won't be voting for Kevin McCarthy tomorrow. He's part of the problem. He's not part of the solution," Good told Fox News on Monday. "There's nothing that indicates to me that he's going to change his pattern since he's been in leadership, where he's part of the swamp cartel."

"I think he's just a shill of the establishment," said Gaetz in an interview with the Daily Caller website last week. "I think that Kevin McCarthy is little more than a vessel through which lobbyists and special interests operate."

Some may also remain wary of his previous stance towards former President Donald Trump following the Capitol riots in 2021, having originally been critical of the then-President, who he said bore responsibility for the attack.