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It might have been the most watched political debate in history, and the emerging consensus among pundits and the instant polls is that Hillary Clinton prevailed over Donald J. Trump on Monday night.
Neither the pundits nor the instant polls are representative of voters over all. But both end up shaping post-debate coverage, whether it’s right or wrong — just as they did four years ago.
Will the result change the race? There’s no way to be sure of how the polls will turn next. But the record of post-debate polling suggests that a victory might not matter quite as much as you might think.
In general, the debates don’t have a huge effect on the polls. Historically, the polls that follow the first presidential debate have differed from the pre-debate polls by an average of just 2.5 points. The biggest shift? Only four points.
Those aren’t huge numbers, although they could be enough to make the race a true dead heat or to give Mrs. Clinton a comfortable lead.
The biggest post-debate shift came in 2012. Mitt Romney erased President Obama’s four-point lead in national polls after the first debate, creating a virtual tie in national polls that lasted for weeks.
Mrs. Clinton has some reason to think that her debate victory might be comparable to Mr. Romney’s, at least in terms of the polls.
A CNN/ORC instant poll found that Mrs. Clinton won by a margin of 62 percent to 27 percent, roughly the same as Mr. Romney’s 67-25 victory.
Yet here’s one reason she might struggle to match Mr. Romney’s bump: His standing might have been a little deflated heading into the 2012 debate. Mr. Obama was just coming off his convention, and Mr. Romney was just coming off his remarks about the “47 percent.” You can also tell a similar story about the previous few debates: George W. Bush in 2000, John Kerry in 2004 and Barack Obama in 2008 were all challengers who fell behind after the incumbent party convention. They all gained after the first debate.
On the other hand, there are far more undecided voters and minor-party voters in 2016 than in 2012. Perhaps they’ll break for Mrs. Clinton.
But even if she does make big gains, it will be fair to wonder whether it actually matters. After all, Mr. Romney lost. In fact, he wound up losing by about the same margin he trailed by in the pre-debate polls: four points. The same story played out in 2000: Mr. Busht took a lead after the debates, and yet Mr. Gore ultimately came back and won the popular vote. In 2004, Mr. Kerry’s gains largely stuck, although he fell short in the election.
So what happened in 2000 and 2012? One possibility is that the shift in the polls was a bit of a mirage. The voters of the winning debater’s party became more excited, and therefore likelier to pick up the telephone — a phenomenon called partisan differential nonresponse. Or they became likelier to be counted by pollsters as likely voters. The reverse might happen to the losing side’s voters. These shifts might affect the polls, but might not have nearly as much of an effect on the underlying chance that voters actually turn out and support their candidates.
If Mrs. Clinton surges, it won’t be clear whether that’s a real shift in the race, even if it leaves Democrats feeling better.
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