As polls close across six time zones on election night, millions will be watching to see who the 47th U.S. president will be. But the wait is likely to be a long one for a few reasons that may not be readily apparent. Election results that look like victory for one candidate at 10 p.m. could shift the other way at 11:30 p.m., or even later. And it may take days to find out who won.

The reasons why election watchers may see sudden changes in Kamala Harris’ or Donald Trump’s standing in the most competitive states is largely influenced by two factors: how absentee ballots are processed and the population density of an area. 

That’s why CBS News’ live coverage will show its whole state models, including exit polls and where there are outstanding votes.

Absentee ballot processing differs by state

It may seem pretty straightforward — only seven battleground states are considered to be in play in the election: Arizona, Nevada, Georgia, Michigan, Wisconsin, North Carolina and Pennsylvania, and these are the states likely to determine the winner. 

But every state has different rules governing when absentee ballots can be processed and counted. In the early hours of election night, there may be an incomplete picture of who’s winning a state because absentee ballots will take longer in some states than others to appear in the count. 

Arizona and North Carolina require all ballots to be received on or before Election Day,  and both states allow ballots to be processed early, which suggests vote totals on election night should include many mail ballots early on after polls close.

Michigan is among the states that begins processing votes early and requires all ballots to be in on Election Day, so the ballot totals are likely to include the absentee ballots.

Nevada also allows early processing of absentee ballots, but ballots may also be received after Election Day; they must be postmarked by Nov. 5 or received in the Election Department by Nov. 8 if they do not have postmarks. In May, Nevada’s secretary of state released guidance aimed at speeding up the release of election results. 

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

Early in-person and mail ballots can begin to be counted at 8 a.m. PT on Election Day, By 6 p.m. PT (9 p.m. ET) on Election Day, county clerks are to provide their first election results to the secretary of state, which expects to release unofficial results as soon as polls close. Previously, no counting began before polls closed, according to the secretary of state.

Wisconsin and Pennsylvania both require all ballots to be received by Election Day, but they largely ban processing and counting of absentee ballots until the morning of Election Day. Some of the mail ballots may appear in initial results, but it will take some time for election workers to count these ballots. About 1.8 million people voted absentee in Pennsylvania this year. 

In 2020, it was Pennsylvania that clinched the presidency for Joe Biden, but the state did not have a projected winner until four days after the election. Absentee voting was higher that year, however, because of the pandemic. Also new this year, Pennsylvania is requiring all ballots to be submitted by Election Day. It is no longer accepting ballots postmarked on Election Day and  received within three days afterward.

Generally, more Democrats have voted early than Republicans, though Republicans made a concerted effort this year to convince their voters to vote by mail or early in person, and appear to have had some success. If they did, it will be in spite of the mixed messages from Donald Trump, who still rails against mail voting.

Urban vs. rural county election results

In the first couple of hours Tuesday night after polls close, there may be a lot of red showing up on the map for Trump and other Republicans, but that may be a function of population density.

Polling precincts in smaller regions often tabulate and report results faster than their urban counterparts simply because there are fewer votes to process. Across the key battleground states in the 2024 presidential election, some of the biggest cities — population centers associated with heavy Democratic majorities — are expected to be among the latest to report full results, experts say. Fulton County, Georgia — home to much of Atlanta — “has a notorious reputation for being slow,” said Emory University professor Andra Gillespie. 

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

“Notorious” is also how a professor in another battleground, Michigan, described the vote processing in that state’s biggest city.

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

“They’re not going to have any of those absentees processed at all before Tuesday,” Dulio told CBS News.

Experts had similar warnings about election returns for major cities in Arizona, Wisconsin and Pennsylvania.

Milwaukee Election Commission Executive Director Paulina Gutiérrez told Wisconsin Public Radio recently she expects that city’s results to be in after midnight.

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

The Supreme Court on Friday left in place a ruling by Pennsylvania’s highest court requiring officials to count provisional ballots made by voters whose mail-in ballots were rejected. In a particularly tight race, that could mean a significant delay.

The processing of those votes could follow a similar pattern, taking longer in Philadelphia and other cities than in less populous regions, says University of Pennsylvania professor Marc Meredith.

“Provisional ballots typically are reported out in smaller counties on Thursday, through Monday in the larger counties,” Meredith said.