US President Donald Trump has formally endorsed embattled Senate candidate Roy Moore, looking past sexual misconduct allegations against the Republican to argue that Mr Moore's vote is needed in Congress.

Key points: Trump makes his strongest endorsement of Roy Moore since accusations surfaced

Trump makes his strongest endorsement of Roy Moore since accusations surfaced Several women accuse Mr Moore of sexual misconduct when they were teenagers

Several women accuse Mr Moore of sexual misconduct when they were teenagers The Alabama Republican says the allegations against him are "simply dirty politics"

Mr Trump tweeted that his party needed Mr Moore to win the Alabama seat in order to secure votes on key issues such as immigration, even as other Republican figures have backed away from the candidate.

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

Mr Moore in reply tweeted that he was thankful for Mr Trump's endorsement and the men later had a phone call.

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Mr Trump's tweet is his strongest endorsement of Mr Moore since the sexual misconduct accusations surfaced against the former Alabama judge last month.

Mr Moore has stood firm in the face of sexual assault and misconduct allegations dating back 40 years.

One woman claimed he touched her through her underwear when she was 14 years old.

Leigh Corfman, now 53, told The Washington Post the incident happened in 1979.

Mr Moore, the Alabama's former chief judge who was a 32-year-old state prosecutor at the time and is now 70, vehemently denied the allegations, calling them "completely false and a desperate political attack".

The Washington Post story also quoted three other women who said Mr Moore dated them when they were between 16 and 18 years old and he was in his early 30s, though none said they had sexual contact with Mr Moore.

The Harvey Weinstein effect has reached into US politics. ( Reuters: Marvin Gentry )

The Alabama Republican's campaign said the allegations were fabricated.

The President has repeatedly slammed Mr Moore's Democratic opponent, former US attorney Doug Jones, but had previously backed Mr Moore's former Republican rival Luther Strange in line with Senate Republicans.

"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!" Trump wrote, referring to Democratic congressional leaders Nancy Pelosi and Chuck Schumer.

While the Alabama Republican Party has also backed Mr Moore for the December 12 election, fellow Republicans in the US Senate have continued to distance themselves despite holding a narrow 52-48 majority in the chamber.

Several women have accused Roy Moore of sexual misconduct. ( ABC News: Adrian Wilson )

Mr Moore returned to the campaign trail last week with a speech calling the allegations against him "simply dirty politics".

Mr Trump was silent for weeks after the allegations against Mr Moore surfaced, and the White House has said he had no plans to make campaign appearances before the December 12 election.

But the US President has, in recent days, raised doubts about the allegations against Mr Moore, and he has agreed to headline a campaign-style rally in Pensacola, Florida — less than 32 kilometres from the Alabama border — just four days before the Alabama election.

White House legislative director Marc Short told CNN that Mr Trump was "making the case to the people of Alabama that it's a factor to consider that not one Democrat has been able to find time to say they support" the GOP tax plan.

Mr Short said Mr Trump's point is that "when allegations arise 38 years later, when Roy Moore has been a very public figure for those 38 years, he's run multiple times statewide in Alabama, the people in Alabama have an opportunity to choose and make some decisions about Roy Moore's character".

"Putting all that together, he's encouraging the people of Alabama to make the right decision," he said.

Wires/ABC