North Carolina has become perhaps the most important state in the election. It has the potential to decide the Senate and the presidency.

Hillary Clinton has led every live-interview survey conducted there since the first presidential debate, even though Mitt Romney won it four years ago. She has a comfortable lead in the surveys taken after the third presidential debate, with Upshot/Siena, NBC/Marist, Quinnipiac, Monmouth and Elon polls showing her ahead by an average of four points.

It’s also a state where the election is well underway. Nearly two million voters — perhaps 40 percent of the electorate — have already cast ballots, and the data from early voting suggests that she has banked a considerable lead. The same data implies that pre-election polls are largely right about the composition of the North Carolina electorate.

This doesn’t mean the pre-election polls will be right. There are a lot of reasons the polls could be wrong besides the composition of the electorate. For instance, undecided voters or the supporters of Gary Johnson could break for Mr. Trump.