US President Donald Trump has recorded a robocall for a Republican Senate candidate accused of sexual misconduct, on the eve of the election.
The president has thrown his political weight fully behind former Alabama judge Roy Moore, who is vying with Democrat Doug Jones for the seat.
But the senior Republican senator for the US state of Alabama has said he will oppose his party’s candidate.
The race remains too close to call a day before Alabamians go to the polls.
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Tuesday’s election could alter the balance of power in the Senate ahead of next year’s mid-term Congressional elections.
The candidate, a former Alabama supreme court judge, denies claims by nine women that he made unwelcome sexual advances, mostly when they were teenagers.
One alleges he molested her when she was 14. Another says he tried to rape her.
His campaign was rolling out an audio message on Monday with a recording of Mr Trump’s voice phoning voters to warn his agenda will be “stopped cold” if Mr Moore loses.
At a rally in Florida on Friday, Mr Trump proclaimed his support for the 70-year-old conservative Christian.
Fitting end to unpredictable year
Analysis by Anthony Zurcher, BBC News
Democrat Doug Jones is ahead by 10 points. No, Republican Roy Moore has a nine-point lead.
The latest batch of Alabama opinion polls are having a difficult time contending with the idiosyncrasies of a low-turnout, mid-December special election, a scandal-plagued Republican candidate in a deeply conservative state and a Democratic base that, while small, may be intensely motivated.
With so many variables at play, it will be tough to draw conclusions about the results of this election – but that won’t deter anyone.
A Jones win would not only be seen as a stern rebuke of President Trump, who has put his name on the line for Moore, it would make Democratic control of the US Senate in 2019 a much more realistic possibility.
If Mr Moore prevails, it’d be a prize for the anti-establishment Steve Bannon faction of the Republican Party and an indication that, when it comes to a seat in the US Senate, ideology matters most for some.
Back when Mr Trump named Senator Jeff Sessions to be attorney general last November, no one imagined all eyes would now be on Alabama. It makes for a fitting end to an entirely unpredictable year, however.
Many other national Republicans, including Senate Majority Leader Mitch McConnell, have distanced themselves from their party’s candidate.
But Richard Shelby, Alabama’s other senator, said on Sunday he could not endorse Mr Moore because he found his accusers “believable” and the state “deserves better”.
The Moore team is banking on voters in the Deep South staunchly conservative heartland refusing to pick a Democrat.
On Monday in Birmingham, Mr Jones, a 63-year-old prosecutor, accused his Republican opponent of hiding.
Mr Moore has kept a low profile in recent days.
But he said in an interview on Sunday with The Voice of Alabama Politics that he “never molested anyone” and he had been a victim of “ritual defamation”.
He plans a rally on Monday night with former White House strategist Steve Bannon.
Mr Jones has spent the past week rallying African-Americans, and he held a series of rallies across the state on Sunday.
Former President Barack Obama and former Vice-President Joe Biden have recorded robo-calls for him.
Tuesday’s winner will take the seat held previously by Trump administration Attorney General Jeff Sessions.
Mr Trump won the state by 28 percentage points over Hillary Clinton in last year’s presidential election.