Alabama voters head to the polls Tuesday to decide the race for U.S. Senate between Republican Roy Moore and Democrat Doug Jones, and the outcome is being closely watched across the nation.

>> Who is Judge Roy Moore?

No Democrat has been elected to the U.S. Senate from Alabama since 1992, and President Donald Trump won the state by nearly 30 percentage points. But allegations that Moore pursued sexual relationships with teenage girls when he was in his 30s have rocked the race. He’s denied the claims.

>> Who is Doug Jones, Democrat facing Roy Moore in Alabama Senate race?

Jones, a former federal prosecutor, has highlighted his opponent’s outspoken conservative views in his bid to energize the state’s Democratic base and flip suburban voters who typically vote for the GOP. Polls show a tight race, though special elections like the one Tuesday are notoriously hard to predict.

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Moore is deeply popular with the state’s evangelical voters, a powerful voting bloc that has enthusiastically supported him in past statewide votes. In the closing weeks of the race, he’s had scattered appearances in rural churches while largely relying on supporters to defend him.

Here are five things to watch with Tuesday’s vote to succeed Jeff Sessions, whose seat became open when Trump tapped him to become U.S. attorney general:

>> Trump tweets support for Roy Moore in Alabama Senate race

1. It’s a big deal. Republicans now control 52 seats in the U.S. Senate, including the one held by Luther Strange, who was appointed to fill Sessions’ seat and was soundly defeated by Moore in September. A Democratic win would mean that Republicans could only afford one “no” vote to pass a Senate measure on party lines, since Vice President Mike Pence would break a 50-50 tie. Some Republicans fear a Moore victory could be equally unsettling for the party. Moore has repeatedly called for Senate Majority Leader Mitch McConnell to step down, and he in turn has withheld his support and funding for the former judge’s campaign. And Democrats would look to tie Moore to a host of GOP candidates seeking office in the midterm elections in 2018, highlighting not only accusations that he’s a sexual predator but also his history of controversial statements.

>> WaPo: Another Roy Moore accuser comes forward with evidence of relationship

2. The bombshell allegations. Allegations against Moore of sexual misconduct involving teenagers while a prosecutor in Etowah County, Alabama, from 1977 to 1982 have threatened to upend the race. Moore has denied the allegations while claiming media outlets and Washington status quo enforcers are trying to derail his campaign. The women have stuck by their stories, and several said they are willing to testify under oath. They have left GOP voters who are concerned by the allegations in a quandary, debating between supporting a candidate accused of being a sexual predator or sending a Democrat to Washington. Some could also stay home on Tuesday or write in a candidate.

3. Alabama’s rural base. The state’s rural Republican base holds outsized sway in Alabama, where grass-roots Republicans have helped ensure that no Democrat has been elected to major statewide office since 2006. But Moore’s margins as a statewide candidate show he has underperformed other Republicans. In 2012, he narrowly won a vote for Supreme Court chief justice even as Mitt Romney carried the state by 22 percentage points. And in his 9-point victory over Strange in the primary, Moore struggled in the affluent, conservative suburbs in Birmingham and Huntsville. Moore has tried to shore up his base by crisscrossing rural areas he hopes to carry by overwhelming victories, and his advisers expect enthusiastic turnout to mark the difference in Tuesday’s vote.

4. The key to a Democratic victory. Jones must rely on a two-pronged strategy to flip the seat. He needs Alabama’s black population – a predominantly Democratic voting bloc that accounts for about 27 percent of the state – to turn out in droves. Jones, who is white, has leaned on African-American supporters, including New Jersey Sen. Cory Booker, to energize black voters in populous areas like Birmingham in the closing days of the race. He has also wooed voters in Republican-leaning suburbs in the outskirts of Birmingham, Huntsville and Mobile in hopes of convincing them to vote across party lines – or not cast a ballot at all. Some suburban voters who have never cast Democratic ballots say they’ve proudly posted Jones signs in their yards.

5. How the election will affect the 2018 elections in other states. For example, although Georgia and Alabama are vastly different states, Peach State strategists are closely watching their neighbor for clues about next year’s elections in Georgia. Like in Alabama, Democrats in Georgia hope to flip independent voters in affluent suburbs who have fled to the GOP. And Republicans in both states see a path to victory through maximizing their advantage in rural areas. U.S. Rep. John Lewis, D-Atlanta, was among the black leaders enlisted to help Jones’ campaign across the state line. And Stacey Evans, a Democratic candidate for governor, has already made clear she intends to weaponize Moore’s campaign. She called on her GOP rivals to disavow Moore’s candidacy. None did so.