The candidates invaded the Philadelphia region this campaign season, demonstrating the importance of the battlefield Pennsylvania.

Vice President Kamala Harris and former president Donald Trump remain stuck in gridlock in national polls and in battleground states, with all eyes on who wins the electoral college.

Here’s what you need to know about the process before Election day.

What is the Electoral College?

As provided in Article II of the Constitution, the President of the United States is elected by the Electoral College, not by popular vote.

Under this system, each state is assigned a number of electoral votes determined by the size of its congressional delegation. The states with the most electoral votes were California (54) and Texas (40).

Experts said the founders debated who should be trusted to choose the nation’s leader, with some urging Congress or state legislatures to make a decision and others arguing for a more democratic vote.

“When they designed our system, they created the Electoral College not because they thought it was perfect, but as a way to get the Constitution passed, and it addressed the concerns of highly populated states and states less populated,” said Rachael Cobb. , professor of political science at Suffolk University, told ABC News.

Although it is not a direct election of the president, they stressed.

“It’s really 51 separate elections,” Barry Burden, director of the Center for Election Research at the University of Wisconsin-Madison, told ABC News. “Each state and the District of Columbia have their own rules for running elections. Then each state rewards its electors separately, and it’s up to the candidates to win a majority of those electors to be elected president.”

How do voters work?

To win the presidential electiona candidate must receive 270 of the 538 total electoral votes.

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Many states have a winner-take-all approach, meaning the candidate who wins the most votes statewide receives all of their electoral votes. Two states, however, have split their electoral votes: Nebraska awards three of its five electoral votes per precinct and Maine awards two of its four votes per precinct.

Voters are generally selected by the state’s political parties.

Although the Constitution does not require electors to vote for the winner of the popular vote in their state, many legislatures have laws in place to prevent an elector from being a rogue elector — or what is called an “elector unfaithful.”

In 2020, some Republican allies of Trump attempted to submit false voter rolls to Congress in order to undermine President Joe Biden’s victory. Several people have been prosecuted for their role in this affair.

Why is this so crucial to this election?

Polls ahead of Election Day show a razor-thin race between Harris and Trump.

It will likely all come down to a handful of key states: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

Some of these states were decided by just 10,000 to 20,000 votes in 2020, and the margins are expected to be just as tight in 2024.

Pennsylvania is considered crucial to the outcome of the race because it has the most electoral votes (19) of any battleground. The state cemented President Joe Biden’s victory in 2020, moving blue after going red for Trump in 2016.

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If Harris wins Pennsylvania, Michigan, Wisconsin and a single electoral vote in Nebraska (which Biden won in 2020), she would reach 270 electoral votes even if she lost the remaining four swing states.

If Trump wins Pennsylvania and holds every state he won in 2020, he will also need to flip Georgia to win.

What happens if there is a tie in the electoral college?

A 269-269 tie is not outside the realm of possibility, although it is unlikely.

This has only happened once under current Electoral College rules: the 1824 presidential election.

In this case, as required by the Constitution, Congress conducted a “conditional election” to determine the winner. A contingent election requires the House of Representatives to choose the winner, with each state having one vote. A candidate would need the support of 26 of the 50 states to be elected.

538 analyzed the different scenarios in which a tie could result between Vice President Harris and former President Trump.

Cobb called the possibility of a tie a “real concern” this year and explained what would likely happen next.

“The House of Representatives is controlled by the Republicans and so the likely outcome of this election, in the event of a tie and if it were to go to the House of Representatives, is that the Republican nominee would win,” Cobb said.

And the popular vote?

“It’s a symbolic victory to win the popular vote, but it doesn’t have to be,” Burden said. “And twice in the last five elections we’ve seen the popular vote go one way and the electoral college go the other. That could well happen this time, too.”

Candidates who lost the popular vote but won the Electoral College include John Quincy Adams in 1824, Rutherford B. Hayes in 1876, Benjamin Harrison in 1888, George W. Bush in 2000, and Trump in 2016.

After the 2016 election, when Hillary Clinton eclipsed Trump by nearly 3 million votes in the popular vote but lost the electoral college, a debate erupted over whether the system needed to change.

Pros and Cons of the Electoral College, According to Experts

Proponents of the Electoral College generally note that it is enshrined in the nation’s founding document and has largely functioned since then.

“Many aspects of the Constitution have not been amended since 1789 and are still in effect. The Electoral College still functions essentially as it was originally intended. So why waste success?” » Burden said.

Burden and Cobb noted that it also forces campaigns to focus on traveling from state to state to meet with voters. Without the need to win over voters, presidential candidates would likely run a more nationalized campaign and focus on densely populated cities while ignoring smaller, rural communities.

The main argument against the Electoral College is that it means that not all votes are equal, as a handful of key states usually determine the winner of an election.

“Another argument is that, essentially, his influence is disproportionate,” Cobb said. “Small states have a disproportionately higher number of electoral votes relative to their population, which means that individual votes in these states carry more weight than in larger states.”

“The idea of ​​one person, one vote has really taken hold in the United States over time, and the Electoral College doesn’t accommodate that idea,” Burden said. “The votes are unequal. Some voters count much more, and others count less. And that is inconsistent with any other type of election in the United States.”

ABC News contributed to this report.