John F. Kennedy won the 1960 presidential election by such a narrow margin that it’s easy to imagine how history might have turned out differently. What if Kennedy had lost a key endorsement or Richard Nixon had better prepared for the first debate? What if it had rained on Election Day?

Unusually heavy rainfall dampens turnout among less reliable voters, according to a 2007 study. These voters tend to favor the Democratic candidate, particularly when a Republican is in power. Nixon needed rain to keep these voters from going to the polls. He didn’t get it.

“In 1960, for instance, it was a remarkably clear day across the entire country. Had it rained that day, we probably would have had a Nixon presidency eight years earlier,” said Brad Gomez, a political scientist at Florida State University and lead author of the study, which simulated every presidential election from 1948 to 2000 under different weather. Researchers also found that rain in Pensacola, Fla., in 2000 may have cost Al Gore the presidency.

The study lent empirical backing to a maxim of campaign politics: Republicans should pray for rain. As recently as 2012, a poll from the Weather Channel found that Barack Obama supporters were more likely than Mitt Romney supporters to be deterred by rain.

But supporters of vice president Joe Biden may not have to worry about rain keeping their voters home.

The forecast for this Election Day calls for essentially no precipitation over the Lower 48, the lone exception being some showers in the typically rainy Pacific Northwest. Furthermore, temperatures will be mild over most of the nation. Highs are forecast to reach at least the 60s everywhere except the Pacific Northwest and Northeast.

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Jay Inslee, left, and Loren Culp, right.
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Even if stormy weather were anticipated, it’s not clear how much it would have benefited Republicans. This year, around 6 in 10 supporters of Biden plan to cast their ballots early or by mail, according to a recent Post-ABC poll.

“Our work shows that people are sensitive to changes in the physical costs of voting,” Gomez said. “Early voting, voting by mail – all of that makes it less likely that rain is going to have an effect on Election Day.”

Among supporters of President President Trump, on the other hand, around 6 in 10 plan to go to the polls on Election Day. But rain would have been unlikely to have kept them home, said Tom Hansford, a political scientist at the University of California at Merced and co-author of the study with Gomez. Incumbent Republicans tend to draw support from more reliable voters, who are less likely to be deterred by a downpour.

“It is not clear to me that rain on Election Day will hurt Trump’s vote share this year,” Hansford said in an email. “But it may help him less than we would normally expect.”

Hansford emphasized that it’s hard to make any confident predictions. There are only a handful of studies on weather and voter turnout, and those studies cover a small sample of presidential elections, so it’s difficult to draw too many conclusions, much less apply those findings to this year’s highly unusual race.

Some research, for instance, suggests that rain influences elections not so much by depressing turnout but by putting voters in a bad mood. When it rains, voters tend to prefer the less risky candidate. That usually means the incumbent or the Republican – though this year it could mean Biden, according to Viviana Rivera-Burgos, a political scientist at the City University of New York. But the overwhelmingly sunny and tranquil weather predicted this year may put voters in a mind-set to select a riskier candidate, perhaps Trump.

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Research also finds that when voters are sufficiently motivated, rain is unlikely to stop them from going to the polls. Rivera-Burgos co-authored a study that looked at voting patterns in 2012 after Hurricane Sandy sparked power outages, shut down subways and forced election officials to move polling sites in parts of New York City. Comparing areas that had seen only a little flooding with areas that had been swamped under several feet of water, researchers found that the storm had little effect on turnout.

Rivera-Burgos said that when voters believe an election will have major consequences for them, their community, or their country, they may be more willing to travel to distant polling sites, stand in long lines or put up with rain. She said Biden voters may feel especially motivated this year, given the pandemic, the recent protests over racism and strong feelings about the Trump presidency.

“Under certain circumstances, personal motivation to vote can override minor and even major costs of voting,” Rivera-Burgos said in an email.

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The Washington Post’s Jason Samenow contributed to this article.