If Moore loses, Republicans will see their majority slip from 52 to 51 in the 100-seat Senate, reducing their margin for maneuver to the bare minimum.

President Donald Trump has made the high-stakes vote a test of his brand of populism by urging loyalists to elect Moore, a 70-year-old ultra-conservative former state chief justice.

Moore, who wants to bring his Christian religious activism to Washington, has for the past month been fighting accusations he fondled two underage girls in the late 1970s when he was a state prosecutor in his 30s.

Moore was asked about the allegations after he and his wife Kayla arrived on horseback to vote at a fire station in the town of Gallant.

"We're done with that," Moore said. "Let's get back to the issues.

"This is a very important race for our country, for our state, and for the future."

The scandal has put a Senate seat from Alabama within reach of Democrats for the first time in a quarter century.

"Roy Moore will be the gift that keeps on giving for Democrats," Senator Lindsey Graham, a Republican from South Carolina, told CNN.

Polls opened across the state at 7:00 am (1300 GMT) and are scheduled to close at 7:00 pm (0100 GMT), with three million voters called to cast ballots.

- Trump, Obama -

To protect the precious Senate seat at stake, Trump ultimately endorsed Moore -- throwing caution to the wind for the mid-term elections, and the party's image.

Jones, the Democratic candidate, received support from former president Barack Obama in a state where the African-American vote could prove decisive.

"This one's serious. You can't sit it out," Obama said in a robo-call to voters.

The latest survey by Fox News put Jones ahead by 10 points although an Emerson poll has Moore ahead by nearly that much. The two are competing to replace Jeff Sessions, whose seat became vacant when Trump appointed him to be US attorney general.

"We feel very, very good about the outcome," Jones said at an early morning rally on Tuesday.

The campaign, the first for a Senate seat since Trump's election, has been memorably virulent, in a state accustomed to rough politics.

Ostracized by many in his own party, Moore has taken a page from Trump's playbook, rejecting the women's allegations as "fake news."

"We're up to our neck in people that don't want change in Washington DC."

His wife Kayla defended her husband and rejected accusations of racism, sexism and anti-Semitism.

"One of our attorneys is a Jew," she said.

- Spending big -

Democrats have invested heavily in the battle. Thanks to an avalanche of donations, they have flooded the airwaves with TV ads, and deployed leading black Democrats to mobilize the African American vote -- about a quarter of the electorate.

Alabama-born NBA legend Charles Barkley appeared at the rally and urged voters to support Jones.

"At some point we got to stop looking like idiots to the nation," Barkley said.

Jones is known for having convicted two Ku Klux Klan members for bombing a black church in Birmingham, killing four African-American girls.

His support for abortion rights, however, is anathema to many conservatives, who may choose to write in another candidate aside from the two on the ballot.