As Close polls On election night, millions of people in six time zones will be watching to see who the 47th US president will be. But for some reasons that may not be immediately apparent, the wait is likely to be a long one. Election results that look like a candidate's victory at 10 p.m. could shift the other way at 11:30 p.m. or even later. And it can take time Days to find out who won.

The reasons why election watchers may see sudden changes in the standing of Kamala Harris or Donald Trump in the most closely contested states are largely influenced by two factors: the way mail-in ballots are processed and the population density of an area.

That's why CBS News' live coverage will show it Whole-state modelsincluding election polls and where there are outstanding votes.

The processing of postal votes varies depending on the federal state

It may seem pretty simple – just seven Battleground states Those considered in play in the election are: Arizona, Nevada, Georgia, Michigan, Wisconsin, North Carolina and Pennsylvania, and those states are likely to determine the winner.

However, each state has different rules regarding when mail-in ballots can be processed and counted. In the early hours of election night, there can be an incomplete picture of who is winning in a state because it takes longer for mail-in ballots to appear in the count in some states than in others.

Arizona And North Carolina They require all ballots to be received on or before Election Day, and both states allow early processing of ballots, suggesting that election night vote totals should include many mail-in ballots early after polls close.

Michigan is among the states that begin processing votes early and require all ballots to be in by Election Day, so the ballot total likely includes mail-in ballots.

Nevada also allows for early processing of mail-in ballots, but ballots can also be received after Election Day; They must be postmarked by November 5th or received at the elections department by November 8th if not postmarked. In May, Nevada's Secretary of State released guidance aimed at expediting the release of election results.

“Earlier release of results will increase transparency, help us combat misinformation and reduce pressure on election officials,” Secretary of State Francisco Aguilar said in a statement at the time.

Counting of early in-person and mail-in ballots can begin at 8 a.m. PT on Election Day. By 6:00 p.m. PT (9:00 p.m. ET) on Election Day, county clerks must submit their initial election results to the Secretary of State, which is expected to release unofficial results once polls close. Previously, no counting had begun before the polling stations closed, said the Foreign Minister.

Wisconsin And Pennsylvania Both require all ballots to be received by Election Day, but largely prohibit the processing and counting of mail-in ballots until the morning of Election Day. Some of the mail-in ballots may appear in the initial results, but it will take time for poll workers to count those ballots. About 1.8 million people voted by mail in Pennsylvania this year.

In 2020, Pennsylvania secured the presidency for Joe Biden, but the state did not have a projected winner until four days after the election. However, the number of postal votes was higher this year due to the pandemic. Also new this year, Pennsylvania requires all ballots to be submitted by Election Day. Ballots postmarked on Election Day and received within three days thereafter will no longer be accepted.

In general, more Democrats have voted early than Republicans, although Republicans have made a concerted effort this year to persuade their voters to vote by mail or vote early in person and appear to have had some success. If so, it will be despite the mixed messages from Donald Trump, who still opposes postal voting.

Election results for urban vs. rural districts

There could be a lot of red on the map for Trump and other Republicans in the first hours Tuesday evening after the polls close, but that could be a function of population density.

Voting districts in smaller regions often produce and report results more quickly than their urban counterparts simply because there are fewer votes to process. In key states contesting the 2024 presidential election, some of the largest cities – population centers with strong Democratic majorities – are expected to be among the latest to announce full results, experts say. Fulton County, Georgia — home to much of Atlanta — “has an infamous reputation for being slow,” said Andra Gillespie, a professor at Emory University.

“Fulton County is also the largest county in the state,” Gillespie said. “It’s not like a district that can turn things around after just 10,000 or 15,000 votes.”

“Infamous” was also how a professor in another battleground, Michigan, described the election process in that state’s largest city.

“Detroit is notoriously slow,” said David Dulio, a professor at Oakland University. And Warren, the state's third-largest city, is also likely to be slower than other areas because it has chosen not to pre-process mail-in ballots. A new law allowed localities in the state to begin processing those ballots up to eight days before the election.

“None of these absentees will be processed before Tuesday,” Dulio told CBS News.

Experts similarly warned about election results for major cities in Arizona, Wisconsin and Pennsylvania.

Milwaukee Election Commission Executive Director Paulina Gutierrez recently told Wisconsin Public Radio that she doesn't expect the city's results to be available until after midnight.

“We are the largest city in the state of Wisconsin,” Gutierrez told the radio station. “We follow the same laws as everyone else here in the state of Wisconsin. We just have a lot more work to do.”

The Supreme Court on Friday left in effect a ruling from Pennsylvania's highest court that required officials to do so count provisional ballots of voters whose mail-in ballots were rejected. In a particularly close race, this could mean a significant delay.

Processing those voices could follow a similar pattern, taking longer in Philadelphia and other cities than in less populated regions, says Marc Meredith, a professor at the University of Pennsylvania.

“Provisional ballots are typically cast on Thursday in smaller counties and by Monday in larger counties,” Meredith said.