For this reason, Mrs. Clinton’s support is unlikely to flag even as her email has again attracted the attention of the F.B.I. Likewise, for committed Republicans, support for Mrs. Clinton is out of the question. In a Florida survey, 84 percent of Trump voters said that Mrs. Clinton should be in prison, and 40 percent said she was a demon.

With polarization, many voters’ preferences have become predictable from their social and cultural characteristics. Even so-called undecided voters are more decided than they realize. It has been suggested that the Republican Party is motivated today not by political conservatism, but a reaction against contemporary life. Trump voters resemble Romney and McCain voters. They are whites who are more likely to be evangelicals who did not graduate from college. Tensions between these groups and elites and minorities limits the range of support that either side’s candidate will receive.

Partisan geography has become more fixed, too. In terms of patterns of relative strength and weakness, the electoral map is more stable than it has been in 50 years. For this reason, talk of Mrs. Clinton’s winning Texas is overblown. If that happens, I promise to eat a bug.

Mr. Trump’s candidacy has revolved almost entirely around emotionally powerful issues like race, immigration and anti-Muslim sentiment. The more you feel a decision in your gut, the less likely it is that you will change your mind. White nationalists think white nationalism is great, but others are repelled. And in a Raycom/Mason-Dixon poll of the Louisiana Senate race, supporters of the white supremacist David Duke favored Mr. Trump over Mrs. Clinton by a margin of 81 percent to 6 percent.

Voter polarization translates easily to extreme legislative bodies. In any district dominated by one party, representatives are determined mainly in primary elections, when turnout is low and the most likely voters are motivated partisans, fulfilling Madison’s fear that “a common passion or interest will, in almost every case, be felt by a majority.” By numerical measures of ideological intensity, from the 1970s to the 1990s, centrism steadily disappeared from the House Republican caucus. Single-party domination has expanded since 2012 because of increased partisan gerrymandering, which eliminated dozens of competitive districts. These trends suggest that partisan gridlock will continue long after Election Day.

Although technology has contributed to polarization, it may also help rescue us. For example, Facebook has automated story selection for its custom news feed; for political news this tends to foster an echo-chamber effect. However, Facebook data scientists have found a better source of diversity: almost 30 percent of hard-news reports originating from friends reflect opposing views. Even better, individuals are likelier to engage with information like this when it is presented in a social context.

For now, we are stuck with an intensely emotional campaign that has been a significant source of stress for more than half of adults. The American Psychological Association has, for the first time, issued tips on dealing with election-related stress. Strong emotional experience reduces mental flexibility, suggesting that when tempers run high, as they have for many voters this season, entrenched support for a party or candidate is more likely. So if you wonder whether there is anyone left to persuade, the answer is probably no. We’re too freaked out.