Which party is in the lead?

That’s a tricky question, in part because who you voted for is not public information, but also because the snapshot of early voting results on any specific day does not necessarily represent what the final result will be. Whoever is in the lead now may not be the party that actually wins.

Right now, Republican-seeming voters have cast more ballots than Democratic-seeming voters, but that’s largely because mail-in ballots are sent to voters weeks before Election Day (to give them time to cast a vote and mail it in.) That means when early voting first opens, mail-in ballots are usually the first votes to be counted.

“The ebb and flow of the election cycle generally is — and it’s not true in every single state but it is true in the national aggregate — Republicans tend to run up the score early with the absentee vote, Democrats come back strong with a pretty massive surge in in-person early voting, then on Election Day the vote tends to be closer to parity with a slight Republican lead,” said Tom Bonier, the chief executive of TargetSmart, a data analytics firm that has collected early voting figures. “Republicans tend to have to play catch up on Election Day.”

Age is also a factor, Mr. McDonald said.

“As we get closer to Election Day, you see that younger people tend to vote later than older people,” he said. “So younger people will enter the electorate as we get into next week.”