“The problems that we’re having with getting representative samples tend to lead us toward people who tend not to be Democrats,” said Scott Keeter, the director of survey research at the Pew Research Center. The most highly regarded pollsters, like those at Pew, have made many adjustments to compensate. But other polls, including many of those informing polling averages and Senate forecasting models, are not nearly as high in quality. Another highly regarded pollster, who requested anonymity, put it more bluntly, calling the new challenges “scary.” Many pollsters are reluctant to say things on the record that might undermine confidence in their own polling; others are unwilling to say anything at all, even to offer basic methodological information.

There are reasons to question whether the problems will be as acute this year, because many of the young and nonwhite voters who pose the biggest challenges to pollsters will most likely stay home in a midterm election. Even if the polls are again biased toward Republicans this year, there are reasons to doubt whether Democrats will retain control of the Senate. The Republicans might have a large enough advantage to withstand another round of modest polling errors.

But the larger concerns will remain, and they might be more significant in 2016 — when younger and nonwhite people are expected to vote in larger numbers.