The race for a Georgia Senate seat is headed to a runoff, ABC News projects. Here's how it works.

No candidate in the Georgia Senate race will receive more than 50% of the vote, ABC News is projecting, which means the state will hold a runoff election between incumbent Democrat Sen. Raphael Warnock and Republican candidate Herschel Walker.

The runoff election will be held on Dec. 6. Georgia state legislators passed a law last year that shortened the runoff period from nine weeks to four weeks.

A runoff is a second election that is held to declare a winner when none of the candidates in the first election were able to garner enough votes to reach the threshold needed to be declared the winner.

Georgia requires a runoff election be held when none of the candidates receive a majority of the votes. Georgia law defines the majority as having 50% of the vote plus one. Candidates in other U.S. states can win an election with just a plurality of the vote, just by garnering the largest number of votes in an election.

Georgia last held a runoff election for Senate seats in 2020 when Democrats Warnock and Jon Ossoff defeated two Republican incumbents in twin runoff elections. This led to a monumental power shift in the control of the U.S. Senate, lending power to Democrats in the body for the first time in a decade.

This year, only one of those two seats is up for reelection. After GOP Sen. Johnny Isakson retired, the governor appointed Kelly Loeffler to fill his seat. But, Loeffler had to run for election in 2020 in order to serve Isakson's remaining two years in office. Loeffler lost that election to Warnock, who is running for his first six-year term in office this year.

How does a runoff work?

When none of the candidates nominated for public office receive a majority of the votes cast, a runoff election is held between the two candidates receiving the highest number of votes.

According to Georgia law, the runoff for a general election is to be held on the 28th day after the general election, in this case Dec. 6. The candidate who receives the highest number of votes cast in the runoff is declared the winner.

If any of the two candidates who received the highest number of votes in the first election withdraws, dies or is found ineligible, the remaining candidates with the two highest number of votes are the candidates in the runoff.

Anyone registered to vote in the state of Georgia can vote in federal, state and local runoff elections. The deadline to register to vote in order to be eligible to vote in the general election runoff was Nov. 7.

Separately, candidates are allowed to request a recount of votes in tight races where the margin is 0.5% or less, but Georgia state law does not require an automatic recount of votes. A candidate would have to make that request within two business days of the election result being certified.