Washington CNN  — 

A system of vote counting known as ranked-choice voting could play a key role in one of the most competitive and closely watched Senate races in the country – the Maine matchup between Republican Sen. Susan Collins and Democratic challenger Sara Gideon.

Ranked-choice voting will come into play in the Maine 2020 Senate race with four candidates appearing on the ballot: Collins, the long-time GOP incumbent fighting to keep her seat, Gideon, the Democratic opponent and state House Speaker, and two independent candidates.

Maine will also be using ranked-choice voting for the presidential race where former Vice President Joe Biden and President Donald Trump will appear on the ballot alongside several other third-party candidates.

The system allows voters to rank candidates in order of preference on the ballot, if they choose to do so. Voters can select a first choice, second choice, third choice, fourth choice, etc. They are not, however, required to rank candidates by preference, and may opt instead to only select a single candidate, who would be their first and only choice.

If one of the four Senate candidates receives a majority of the vote – or 50% plus one – on Election Night that candidate will win the race outright without ranked-choice tabulation taking effect.

But, if that does not happen, then the lowest-ranked candidate of the four will be eliminated. If that candidate’s supporters ranked other preferences, their votes will then be assigned to their second-ranked choice. That process will repeat until only two candidates remain with the winner determined in the final round.

Polls have shown an extremely tight race between Collins and Gideon, potentially increasing the likelihood that vote counting ends up moving into multiple tabulation rounds. If that occurs, it may give an advantage to Gideon. That’s because Lisa Savage, a progressive independent candidate who is also running in the race, has encouraged supporters to rank Gideon second.

The close race comes with control of the Senate at stake in Tuesday’s elections as Democrats nationwide attempt to win back the majority in the upper chamber. Democrats have made Maine one of their top targets in the battle for the Senate and Collins is attempting to defend her seat in a state that Hillary Clinton won in the 2016 presidential election.

The Maine race isn’t the only Senate election where the outcome may not be known on election night: A pair of Georgia Senate races could end up in January runoffs, a scenario that might mean it’s not clear for weeks which party will next control the Senate.

When ranked-choice voting tabulation rounds are initiated, couriers are sent around the state to either collect actual ballots or memory devices and bring them to a secure location at the State Capitol in Augusta. There, high speed tabulators process the ballots, and memory devices are uploaded. A secure computer is then used to determine the results by applying the ranked-choice voting rules.

That process can take between one-and-a-half to two weeks to complete, so if no candidate wins outright with a majority on November 3, it will not be clear on Election Night who won the closely watched Senate race. That outcome that could inject uncertainty into the fight for control of the Senate.

Democrats have made Maine one of their top targets in the 2020 fight to win a Senate majority and Collins is attempting to defend her seat in a state that Hillary Clinton won in the 2016 presidential election.

The high-stakes consequences of ranked-choice voting were on display in Maine in the past election cycle.

In 2018, Democrat Jared Golden defeated Republican Rep. Bruce Poliquin in Maine’s 2nd District House race – flipping the seat after the state calculated its ranked-choice voting results.

The result marked the first time in the nation’s history a federal race had been decided by ranked-choice voting.

CNN’s Lauren Dezenski and Eric Bradner contributed to this report.