For example, in recent days, Mr. Trump has campaigned in New Mexico, a state he has no chance of winning. Candidates can get more money and adjust their message, but the one thing they can’t do is make more time; every wasted hour in a noncompetitive state is a grave error. Mrs. Clinton continues to go on the offensive in states like Arizona, where the race is close.

“Big data” is a buzzword, but that concept is outdated. Campaigns have entered the era of “little data.” Huge data sets are often less helpful in understanding an electorate than one or two key data points — for instance, what issue is most important to a particular undecided voter.

With “little data,” campaigns can have direct, highly personalized conversations with voters both on- and offline, like an ad on a voter’s Facebook page addressing an issue the voter is passionate about. In 2016, we see that online political engagement rates (especially for young voters) are at a historic high.

This is why campaigns no longer pay much attention to public polls, which often use conversations with just a few hundred people to make predictions about the entire electorate. Getting a truly representative sample has become ever more difficult because of the growing percentage of households with only cellphones, the number of voters who prefer to speak a language other than English, and the difficulty in contacting younger voters, who generally don’t have landlines.

Smart campaigns can use “little data” to solve these problems. They look at public data sets that list each registered voter’s name, address, party registration and election participation history. By analyzing these voter files, they develop an accurate idea of the makeup of the electorate. Rather than rely on voters’ (frequently inaccurate) estimates of their own likelihood to vote, these campaigns look at their turnout record, thus getting a very precise idea of who “likely voters” really are. The media outlets that conduct national polls usually can’t afford to do this.

Even the best public polling can tell us only about broad categories of voters — not much help guiding a personalized conversation via text message, as is more common with voters in 2016 than it was even in 2012.