(NewsNation) — The contentious and closely watched Georgia Senate race between Democratic incumbent Raphael Warnock and Republican challenger Herschel Walker remains undecided. Although data from Decision Desk HQ showed Warnock holding a slight lead over Walker as of Thursday afternoon, neither candidate had garnered the majority of votes necessary under Georgia’s voting system.

That means another election will take place in December, this time between just the two candidates who received the most votes.

Here’s what you need to know:

What is a runoff election and how does it work?

A runoff election is a second election that determines the winner of a race where no candidate in the first election met the requirements to claim a victory.

In Georgia, a candidate must win the majority — more than half — of all of the votes cast. In particularly close races like the one between Warnock and Walker, a runoff election is held when no candidate wins an outright majority.

It works like another election and the person with the most votes wins.

How did we get here?

With more than 99% of the vote counted, Warnock is narrowly leading against Walker, a former football star endorsed by President Donald Trump. But Warnock has not yet reached the 50% threshold needed to avoid a runoff, according to data from Decision Desk HQ, which called the race for a runoff on Wednesday.

When will the runoff election happen?

According to Georgia’s election code, the runoff election would take place on the 28^th day after the original election. In this case, that would make Dec. 6 the date of the Warnock vs. Walker runoff.

Who can vote?

Georgia voters must be registered by Nov. 7 to be considered eligible to vote in the upcoming runoff.

The Washington Post has reported that in Georgia runoff elections, military and overseas voters will be sent a special absentee runoff ballot along with their regular absentee ballot before the primary or general election takes place.

The runoff ballot allows those voters an option to rank their choices of all possible candidates for each office and will be used in the case of a runoff.

What’s notable about this race?

Most significantly, the outcome of the race will impact the balance of the U.S. Senate and whether it’s controlled by Democrats or Republicans.

Of the 100 Senate seats, 35 were up for election during the midterms, including one special election. Democrats currently have an effective majority in the chamber, with a 50-50 split and Vice President Kamala Harris wielding the tie-breaking vote. Democrats held 14 seats and Republicans held 21 seats up for election in the 2022 midterms.

Warnock, a pastor at Ebenezer Baptist Church, the Atlanta church civil rights leader Martin Luther King Jr. led, won his seat in a runoff in 2021.

It was one of two Senate seats up for grabs in the formerly reliably Republican state. In a political upset, both Democrats won their races, handing their party the barest of majorities in the chamber.

Those races were the most expensive congressional elections to date at that time, with the special election between Warnock and his Republican opponent, businesswoman Kelly Loeffler, drawing nearly $363 million, according to Open Secrets.

So far, Warnock’s campaign has spent $135.8 million, while Walker’s campaign has spent $32.4 million, according to data from the Federal Election Commission. Millions in outside spending are also expected to flood the state.

Reuters contributed to this story.