What is a 'contingent election' and what happens if the Biden-Trump vote ends in a tie?
The people vote for their next president today, 3 November, but what happens if after all the millions of votes are counted it's a dead heat?
Today, 3 November, is Presidential Election day, with the people of America choosing between incumbent Donald Trump and former vice-president Joe Biden for who will be the next person to lead the country.
While Biden holds a healthy lead in the polls nationwide, the election race looks to be far tighter, with the electoral college system meaning the two candidates' fortunes rest with the outcomes in certain key battleground states, rather than a simply overall majority.
Here we look at what could happen if there are delays to the final count, if there are perceived irregularities, or if there is a tie between Donald Trump and Joe Biden.
US presidential election: the vote count
The candidate who wins each state's popular vote typically earns that state's electors. This year, the electors meet on 14 December to cast votes. Both chambers of Congress will meet on 6 January to count the votes and name the winner.
Typically, governors certify the results in their respective states and share the information with Congress.
But some academics have outlined a scenario in which the governor and the legislature in a closely contested state submit two different election results. Battleground states of Pennsylvania, Michigan, Wisconsin and North Carolina all have Democratic governors and Republican-controlled legislatures.
According to legal experts, it is unclear in this scenario whether Congress should accept the governor's electoral slate or not count the state's electoral votes at all.
While most experts view the scenario as unlikely, there is historical precedent. The Republican-controlled Florida legislature considered submitting its own electors in 2000 before the Supreme Court ended the contest between Bush and Gore. In 1876, three states appointed "dueling electors," prompting Congress to pass the Electoral Count Act in 1887.
Under the act, each chamber of Congress would separately decide which slate of "dueling electors" to accept. As of now, Republicans hold the Senate while Democrats control the House of Representatives, but the electoral count is conducted by the new Congress, which is sworn in on 3 January.
If the two chambers disagree, it's not entirely clear what would happen.
The act says that the electors approved by each state's "executive" should prevail. Many scholars interpret that as a state's governor, but others reject that argument. The law has never been tested or interpreted by the courts.
Ned Foley, a law professor at Ohio State University, called the ECA's wording "virtually impenetrable" in a 2019 paper exploring the possibility of an Electoral College dispute.
Another unlikely possibility is that Trump's Vice President Mike Pence, in his role as Senate president, could try to throw out a state's disputed electoral votes entirely if the two chambers cannot agree, according to Foley's analysis.
In that case, the Electoral College Act does not make clear whether a candidate would still need 270 votes, a majority of the total, or could prevail with a majority of the remaining electoral votes - that is, 260 of the 518 votes left after removing Pennsylvania.
"It is fair to say that none of these laws has been stress-tested before," Benjamin Ginsberg, a lawyer who represented the Bush campaign during the 2000 dispute, told reporters in a conference call on 20 October.
The parties could ask the Supreme Court to resolve any congressional stalemate, but it's not certain the court would be willing to adjudicate how Congress should count electoral votes.
US election: what is a contingent election?
A determination that neither candidate has secured a majority of electoral votes would trigger a "contingent election" under the 12th Amendment of the Constitution. That means the House of Representatives chooses the next president, while the Senate selects the vice president.
Each state delegation in the House gets a single vote. As of now, Republicans control 26 of the 50 state delegations, while Democrats have 22; one is split evenly and another has seven Democrats, six Republicans and a Libertarian.
A contingent election also takes place in the event of a 269-269 tie after the election; there are several plausible paths to a deadlock in 2020.
Any election dispute in Congress would play out ahead of a strict deadline - Jan. 20, when the Constitution mandates that the term of the current president ends.
Under the Presidential Succession Act, if Congress still has not declared a presidential or vice presidential winner by then, the Speaker of the House would serve as acting president. Nancy Pelosi, a Democrat from California, is the current speaker.