Part of the problem stems from the polling process itself. Getting reliable samples of voters is increasingly expensive and difficult, particularly as Americans go all-cellular. Response rates have plummeted to 9 percent or less.

The alternative, online surveys, may have promise, but as the data journalist Nate Cohn has pointed out, the inability to ask nuanced questions can distort results. Many surveys of both varieties do not ask questions in Spanish, muddying the results among Latino Americans. Question wording and question order can have big effects on outcomes as well.

With low response rates and other issues, pollsters try to massage their data to reflect the population as a whole, weighting their samples by age, race and sex. But that makes polling far more of an art than a science, and some surveys build in distortions, having too many Democrats or Republicans, or too many or too few minorities. If polling these days is an art, there are a lot of mediocre or bad artists.

When polling aficionados see results that seem surprising or unusual, the first instinct is to look under the hood at things like demographic and partisan distributions. When cable news hosts and talking heads see these kinds of results, they exult, report and analyze ad nauseam. Caveats or cautions are rarely included.

To be sure, with a few exceptions, like the Democratic primary in Michigan, polls have done a good job of predicting the outcomes of the presidential primaries. For the general election, however, the challenge confronting the pollsters is different and in some ways greater.

The demographic composition of the American electorate is changing rapidly, becoming more racially diverse with every election cycle, and these changes are most evident among the youngest generation of voters. Because there is a deep racial and generational divide between the parties, underrepresenting younger voters and racial minorities can seriously bias poll results. This problem is likely to be exacerbated by the presence at the top of the Republican ticket of Mr. Trump, whose electoral strategy is based on appealing to older white voters.