“The president had a positive call with Judge Roy Moore during which they discussed the state of the Alabama Senate race and the president endorsed Judge Moore’s campaign,” said White House principal deputy press secretary Raj Shah.

Moore’s campaign team said he was honoured to have received the president’s endorsement, in a call placed from on board Air Force One.

“President Trump knows that the future of his conservative agenda in Congress hinges on this election,” Moore was quoted as saying. “I look forward to fighting alongside the president to strengthen our military, secure our border, protect our gun rights, defend the sanctity of life, and confirm conservative judges to courts around this nation.”

In spite of a history of controversy stemming from his tenure on Alabama’s supreme court, Moore had been a strong favourite to win election before the allegations against him were first reported by The Washington Post.

Most of the reported allegations relate to when Moore was a prosecutor in his mid-30s and sought to pursue relationships with teenage girls.

The Alabama race, which is being held to replace Jeff Sessions, who Trump named US attorney general, has national repercussions because Republicans hold a slim 52-48 majority in the Senate. A win for Jones would make it 51-49.

Earlier Monday, Trump cited Democrats’ opposition to his legislative agenda – and Moore’s potentially decisive Senate vote on everything from immigration to abortion – as reasons to support him.

“Democrats refusal to give even one vote for massive Tax Cuts is why we need Republican Roy Moore to win in Alabama,” Trump tweeted.

“We need his vote on stopping crime, illegal immigration, Border Wall, Military, Pro Life, V.A., Judges 2nd Amendment and more. No to Jones, a Pelosi/Schumer Puppet! Look at your 401-k’s since Election. Highest Stock Market EVER! Jobs are roaring back!” he added, referring to an employer-sponsored retirement savings plan.

On the national level, some Republicans are nonetheless horrified at the idea of seeming like a party that defends men who assault women.

Nonetheless, the Republican National Committee late on Monday chimed in with its support for Moore.

“We stand with the president,” a senior RNC official told Breitbart News.

Trump appears to have calculated that the loss of a Senate seat would be too politically costly for him.

The US president will be in Pensacola, Florida late Friday for a campaign-style rally – not far from the Alabama border, raising the possibility that he could end up directly addressing some Alabama voters.

Moore has denied the allegations against him, and said they are politically motivated.

One of his accusers, Leigh Corfman, now 53, said that when she was 14, Moore took her into his house in the woods near Gadsden, Alabama, removed her shirt and pants, and fondled her over her bra and underpants.

Allegations of sexual harassment have plagued both of America’s main political parties in recent weeks.

Democrat John Conyers, a celebrated civil rights activist who is the longest-serving member of Congress, announced he was stepping down from a leadership position last month as he battles harassment claims from members of his staff.

Trump himself has been accused of misconduct by several women, ranging from groping to unwanted advances and kissing, though these did not prevent his election last year.

Jones is currently leading Moore by 2.6 per cent, according to an average of opinion polls cited by RealClearPolitics – within the statistical margin of error.