President-elect Joe Biden is expected to take a major step toward cementing his Nov. 3 victory over President Donald Trump on Monday, when the 538 members of the US Electoral College gather to cast their official ballots for president.

Here is what to expect.

WHAT IS HAPPENING ON MONDAY?

The winner of the US presidential election is determined not by the popular vote but through a system called the Electoral College, which is mandated in the Constitution and allots "electoral votes" to states and the District of Columbia based on their congressional representation.

Before the election, state-level leaders of the two major parties selected people to serve as "electors."

Technically, Americans are casting votes for those slates of electors, not the candidates themselves.

Those individuals are typically party loyalists who have pledged to support the candidate who got the most votes in their state.

Through a flurry of unsuccessful lawsuits and conversations with state lawmakers, Trump, who has refused to concede and made unsubstantiated claims of widespread election fraud, has sought to have slates of electors pledged to him certified in states won by Biden. That effort has failed.

There are 538 electoral votes, meaning 270 are needed to win the election.

Most electors are not household names, but the electors this year include Hillary Clinton, the former secretary of state and 2016 Democratic presidential nominee, and Georgia Democrat Stacey Abrams, a former candidate for governor in that state.

Electors meet at a time and place selected by their state's legislature. Nevada is meeting virtually this year because of the coronavirus pandemic. Most states will livestream the ceremonies.

Electors will sign certificates showing their votes, which are sent to government officials including Vice President Mike Pence. Those certificates are paired with ones signed by governors showing the popular vote tallies, which have already been certified by all 50 states and the District of Columbia.

WHAT HAPPENS NEXT?

Electoral votes will be officially tallied by a newly seated Congress on Jan. 6, in a special joint session that Pence will preside over.

At that point, the election is officially decided. Biden would be sworn in as president during an inauguration ceremony at noon on Jan. 20 — a time set in the Constitution.

CAN ELECTORS DEFY THE POPULAR VOTE?

Yes, but that is a rare occurrence.

In 2016, seven of the 538 electors cast ballots for someone other than their state’s popular vote winner, an unusually high number.

Thirty-two states and the District of Columbia have laws intended to control rogue electors, or "faithless electors." Some provide a financial penalty for a rogue vote, while others call for the vote to be canceled and the elector replaced.

It would be nearly impossible for faithless electors to hand a last-minute victory to Trump.

Biden, a Democrat, won 306 electoral votes to Trump's 232, so the Republican president would need 38 electors to defect.

COULD CONGRESS REFUSE TO ACCEPT BIDEN'S ELECTORAL VOTES?

It is theoretically possible, but such a move is extremely unlikely to work because Democrats control the House of Representatives.

A U.S. law called the Electoral Count Act allows individual members of the House and Senate to challenge the results during the Jan. 6 special session — a rarely used procedure.

Any objection to a state’s results must be backed by at least one House member and one senator.

The two chambers would then separate to debate the objections before voting on whether to reject the state’s results.

An objection must pass in both chambers by a simple majority. If the vote followed party lines, Republicans would fail to block Biden’s victory because Democrats control the House.

Trump would face roadblocks even in the Senate, which is currently controlled by his party. Several Republican senators have called Biden the president-elect, and Republican Senator Mitt Romney, his party's 2012 presidential nominee, told reporters it would be "madness" for Congress to override the Electoral College.