The president has thrown his political weight fully behind Republican former Alabama judge Roy Moore, who is vying with Democrat Doug Jones for the seat.

Mr Moore denies claims by several women that he made unwelcome sexual advances, mostly when they were teenagers.

The race remained too close to call in opinion polls.

A win for the Democrats could reduce the power of the Republican party, who currently hold just a two-seat Senate majority, to pass legislation ahead of next year's mid-term Congressional elections.

While it is already difficult to pass legislation in the Senate, losing another seat would be a blow for Mr Trump, as approval must be reached by the majority of voting members.

However a win for Mr Moore would be a victory for the president, who has been very vocal about his support for the Republican candidate.

Mr Trump's relationship with fellow Republicans has been at times tense, with some launching attacks on both his character and his presidency.

Mr Trump has previously blamed his own party for legislative failures.

Tuesday's winner will take the seat held previously by Attorney General Jeff Sessions.

A big contingent of very enthusiastic Roy Moore fans at the rally last night. We can’t have a Pelosi/Schumer Liberal Democrat, Jones, in that important Alabama Senate seat. Need your vote to Make America Great Again! Jones will always vote against what we must do for our Country.

What are the allegations against Moore?

While endorsing Mr Moore, Mr Trump has pointed to the former Alabama supreme court judge's repeated denial of the allegations against him.

One woman alleges Mr Moore molested her when she was 14. Another says he tried to rape her.

At a rally on Monday night, Mr Moore reiterated his denials, again questioning why his accusers had kept quiet for 40 years while he held various political offices.

Speaking alongside Mr Trump's former chief strategist, Steve Bannon, in front of a crowd which chanted the president's slogan "Drain the Swamp", Mr Moore drew heavily from the Bible.

"I want to make America great again with President Trump," he said. "I want America great, but I want America good and she can't be good until we go back to God."

Mr Moore was joined at the rally by his wife Kayla who said that separate allegations last week that her husband was anti-Semitic were "fake news".

"One of our attorneys is a Jew, we have very close friends who are Jewish," she said.

Mr Moore's campaign rolled out an audio message earlier on Monday with a recording of Mr Trump telling voters his agenda would be "stopped cold" if Mr Moore lost.

At a rally in Florida on Friday, Mr Trump proclaimed his support for the 70-year-old conservative Christian who served in the Vietnam War.

More about: #Trump   #Alabama