Jae C. Hong / AP

One might say there are three keys to evaluate polls in presidential candidates: average, average, and average. That is, don’t get caught up in any single poll, but rather rely on a running mean of polls, whose cumulative average is far closer to the truth of the electorate’s preference than any single survey.

But this tripartite rule is often ignored by the press and public, who care not only about the truth, but also about stories. As Adrienne LaFrance reminded Atlantic readers yesterday, every great story has a bit of movement. Rising is a good story (Horatio Alger), falling is a good story (King Lear), and rising-then-falling is a great story (Icarus).

Stasis is the worst story in the world, but unfortunately, the opinions of the electorate are not a crowd-sourced John Grisham novel. Barack Obama led John McCain and Mitt Romney for basically the entire 2008 and 2012 campaigns, but that didn’t stop the press from dramatizing every poll that showed a slight narrowing of the gap. Similarly Hillary Clinton has led Donald Trump by a comfortable margin for a while, but that doesn’t prevent the press from freaking out when news like today's McClatchy-Marist poll—which put Clinton's lead at a perilous 3 points—whets the media's appetite for a little Icarus narrative.

So here’s another simple rule to evaluate presidential polls going forward. Call it the 80-55-40 Rule: Since 1980, no Democratic presidential candidate has won less than 80 percent of blacks; 55 percent of Hispanics; or 40 percent of whites. (A tiny exception: In 1992, Clinton got 39 percent of the white vote, and Bush got 40 percent; Perot did exceptionally well among white voters.)

Here are the exit-poll breakdowns of the five most recent elections.

It would be nice to call this the 80-60-40 Rule, but history doesn’t always cooperate with heuristics. Democrats won less than 60 percent of Hispanic support twice since 1980, with Carter in 1980 (56 percent) and Kerry in 2004 (56 percent).

How should one use the 80-55-40 rule? Look at today’s poll, which has Clinton winning just 52 percent of Latinos. Following the 80-55-40 rule, this should raise a tiny red flag. Donald Trump has accused a Mexican American judge of putting his family’s heritage above the law, repeatedly promised to build a wall to keep out Mexicans, opened his candidacy by declaring “they’re rapists, and some, I assume, are good people,” and reached out to undecided Latinos by posing next to a taco salad.

Is it possible that Trump could break with recent precedent, and win a historically high share of Hispanic voters? Yes, under certain quantum-theory conditions, all sorts of things are possible: Time can move backwards, tornados can build neighborhoods from pieces of homes floating in the sky, and a cracked egg can leap into its original unbroken shell. But unless you think that “the Hispanics” love Trump as much as he swears he loves them, you might want to question the poll.

The 80-55-40 Rule doesn’t mean that you should automatically reject every poll showing Hillary Clinton receiving, for example, just 75 percent support from blacks. You can choose to believe that number. But such a belief must live side-by-side with recent history. Since 1980, no Republican candidate has won more than 12 percent of the black vote. So for Trump to win, say, 20 percent of black voters, it means that a candidate drawing unprecedentedly public support from white nationalists is doing twice as well among black voters as any GOP candidate in modern history.

Does that make any sense? Well, it’s a democracy. You decide.

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