Mr Moore has been surrounded in controversy and dogged by historical allegations of sexual misconduct towards teenagers, including one aged just 14 during the campaign.

And the former Judge is drawing support from Donald Trump's former political strategist Steve Bannon, who campaigned with him as voters went to the polls today.

Mr Moore – who famously rode his horse to an election rally before pulling out a gun on stage – is being tipped to win a tight contest, having secured the endorsement of the US President, despite his opponent, Doug Jones, relentlessly hammering him in TV adverts in the closing stages of the campaign.

Today he travelled to his local voting station on horseback, galloping over a hill in bizarre scenes. 

The Alabama contest has divided the Republican Party.

The US President offered strong backing during a phone call last week but other Republicans, including Senate Majority Leader Mitch McConnell, have distance themselves from the candidate.

Mr Moore has been accused by multiple women of pursuing them when they were teenagers and he was in his 30s, including one woman who said he tried to initiate sexual contact with her when she was 14. Moore has denied any misconduct.

On the eve of Tuesday's election, Mr Moore was joined on the campaign trail by Mr Bannon, Trump's former chief strategist and an executive at the right-wing Breitbart News site.

Mr Bannon framed the Alabama election as a showdown between establishment elites and populist power and blasted Republicans who declined to support Moore.

He said: "There's a special place in hell for Republicans who should know better."

Mr Moore - who believes homosexual activity should be illegal and has argued against removing segregationist language from the state constitution – told the rally on Monday: "I want to make America great again with President Trump. I want America great, but I want America good, and she can't be good until we go back to God."

Republican former Secretary of State Condoleezza Rice, an African-American who grew up in Alabama, issued a statement on Monday calling the special election "one of the most significant in Alabama's history."

She urged Alabama voters to "reject bigotry, sexism, and intolerance."

A Fox News Poll conducted on Thursday and released on Monday put Mr Jones ahead of Mr Moore, with Mr Jones potentially taking 50 percent of the vote and Mr Moore 40 percent. Fox said 8 percent of voters were undecided and 2 percent supported another candidate.

However, an average of recent polls by the RealClearPolitics website showed Mr Moore ahead by a slight margin of 2.2 percentage points.

No Democrat has held a US Senate seat from Alabama in more than 20 years. In 2016, Mr Trump won the state by 28 percentage points over Democrat Hillary Clinton.

If Mr Jones wins on Tuesday, Republicans would control the Senate by a slim 51-49 margin, giving Democrats much-needed momentum ahead of the November 2018 congressional elections, when control of both chambers of Congress will be at stake.

Mr Moore's campaign has cast Jones as a liberal out of step with Alabama voters, seizing on the Democrat's support of abortion rights.