Nonresponse bias can be a hard problem to solve. Response rates to even the best telephone polls are in the single digits — in some sense, the people who choose to respond to polls are unusual. Trump supporters often have lower civic engagement and social trust, so they can be less inclined to complete a survey from a news organization. Pollsters are attempting to correct for this problem with increasingly aggressive data-massaging techniques, like weighing by educational attainment (college-educated voters are more likely to respond to surveys) or even by how people say they voted in the past. There’s no guarantee any of this will work.

If Mr. Trump does beat his polling, there will have been at least one clear sign of it: Democrats no longer have a consistent edge in party identification — about as many people now identify as Republicans.

There’s also the fact that Ms. Harris is running to become the first female president and the second Black one. The so-called Bradley effect — named after former Los Angeles Mayor Tom Bradley, who underperformed his polls in the 1982 California governor’s race, for the supposed tendency of voters to say they’re undecided rather than admit they won’t vote for a Black candidate — wasn’t a problem for Barack Obama in 2008 or 2012. Still, the only other time a woman was her party’s nominee, undecided voters tilted heavily against her. So perhaps Ms. Harris should have some concerns about a “Hillary effect.”

Wendell Husebo is a political reporter with Breitbart News and a former RNC War Room Analyst. He is the author of Politics of Slave Morality. Follow Wendell on “X” @WendellHusebø or on Truth Social @WendellHusebo.