Many votes have already been cast, and the task of counting mail-in and absentee ballots is likely to delay final results in several states, including those where absentee ballots could prove to be important in close races.

Though 35 seats are up for election, control of the Senate is likely to be decided in a handful of tight contests in states such as Pennsylvania, where all votes may not be counted on election night, and Georgia, where a potential runoff could be in play.

Here are key states where final Senate results could take days.

Pennsylvania

In what may be the most-watched Senate race in the country, Democratic Lt. Governor John Fetterman is facing Republican Dr. Mehmet Oz to fill Pennsylvania’s open seat. The result could be crucial to controlling the Senate.

Polls close at 8 p.m. E.T., and votes cast on election day are expected to be reported on Tuesday night. However, election officials were not allowed to process absentee ballots before Tuesday morning, meaning the final results could take days to arrive.

Leigh M. Chapman, acting secretary of the Commonwealth of Pennsylvania, told reporters during a Zoom call in October that she was asking for “patience.”

“Official results will be available within a few days,” Chapman said. “This delay does not mean anything nefarious is happening. It simply means that the process is working as it is designed to work in Pennsylvania and that election officials are doing their job to count every vote.”

However, the state’s most-populous counties are expected to have most of their counting done by Wednesday morning.

Arizona

Democratic Senator Mark Kelly is slightly favored to win the Arizona Senate race against Republican Blake Masters, according to poll tracker FiveThirtyEight’s analysis, but the complete results may not be known for days.

Polls will close at 9 p.m. E.T., and a large batch of results from mail-in and absentee votes are expected to be announced at 10 p.m. Votes cast on election day will be announced throughout the night, but it could take days to count the remaining absentee ballots.

It’s possible the winner will become clear on Tuesday night, but if the election is particularly close, Arizona may have to wait until all votes are counted to know who the next senator will be. It has sometimes taken a week or more for all the results to come in.

Nevada

Democratic Senator Catherine Cortez Masto is facing a tough re-election fight against Republican Adam Laxalt in the Nevada race, which FiveThirtyEight’s final analysis says is a dead heat.

Polls will close at 10 p.m. E.T., but Nevada will not release any results until after the last voter in line has cast their ballot. During the 2022 primary, that occurred at 12.42 a.m. E.T.

Full results are likely to take even longer as the state accepts mail-in ballots postmarked by election day if they arrive by November 12, while results will continue to come in until November 15.

Ohio

Democratic Representative Tim Ryan is hoping to defeat Republican and author J.D. Vance in Ohio, but the final FiveThirtyEight analysis of the race finds the GOP candidate favorite to win.

A Democratic win would be a significant victory for the party and a major upset, but full results could take some time. Polling will close at 7.30 p.m. E.T. and, not long after that, Ohio counties will report the early and mail-in votes counted up to that point, while results from election-day ballots will be reported in the following hours.

However, full results could take time as Ohio counts mail-in ballots postmarked before November 7 if they arrive by November 18. It remains to be seen if that will have a major impact in a race that polling suggests strongly favors Vance.

Georgia

The Senate race in Georgia may take time to decide not because of a long process on election night, but because a close result will require a runoff election.

Democratic Senator Raphael Warnock is running in a tight race against Republican Herschel Walker in a state that has long been seen as red. FiveThirtyEight rates Walker as slightly favorite to win the race, but polling has painted a mixed picture.

Polls will close at 7 p.m. E.T., and counting is expected to be quick. Georgia’s new election law requires all counties to finish counting votes by 5 p.m. on Wednesday, while also affording them more time to count absentee ballots.

However, if neither candidate wins 50 percent of the vote, the race will go to a runoff, which will be held on December 6. That would mean the Georgia election, and possibly control of the Senate itself, would take almost another month to decide.

While candidates may be in for a long night across the nation, many crucial results may not be known for several days.