The polls have closed in Alabama, where a firebrand Republican conservative is battling for a Senate seat against a Democrat hoping for a huge upset.
President Donald Trump's populist brand will be tested after he backed Roy Moore, who denies allegations of sexual misconduct with teenage girls.
Much of the Republican establishment has distanced itself from the 70-year-old former Alabama judge.
The race between Mr Moore and Democrat Doug Jones is too close to call.
The Republican candidate has said homosexual activity should be illegal and argued against removing segregationist language from the state constitution.
But it is sexual abuse claims against him by a number of women, some when they were teenagers, that have made Washington conservatives baulk, reports BBC.
One accuser alleges Mr Moore molested her when she was 14.
The scandal has put an Alabama Senate seat within reach of Democrats for the first time in more than two decades.
Who is Moore's opponent?
Mr Jones, a 63-year-old former prosecutor, denies opponents' claims he will be a "puppet" of the Democratic congressional leadership.
He is lauded for helping convict two Ku Klux Klan members who bombed a black church in 1963 in Birmingham, killing four girls.
But Mr Jones' support for abortion rights is toxic to many Christian conservatives in Alabama.
After casting his ballot on Tuesday morning, he predicted: "I don't think Roy Moore is going to win this election."
Former President Barack Obama has recorded an automated phone message for Mr Jones.
"This one's serious," Mr Obama told voters in his call. "You can't sit it out."