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Clinton Probably Finished Off Trump Last Night

I’m not sure I need to tell you this, but Hillary Clinton is probably going to be the next president. It’s just a question of what “probably” means.

Clinton went into the final presidential debate on Wednesday with a lead of about 7 percentage points over Donald Trump. And according to the only two scientific polls we’ve seen, voters thought that Clinton won the debate. Occasionally, the initial reaction to a debate can differ from the way it’s perceived days later. But in this case, the morning headlines, which focused overwhelmingly on Trump’s refusal to say whether he’ll accept the election results, are potentially worse for Trump than the debate itself. In YouGov’s poll of debate watchers, 68 percent of voters said they think the candidates should pledge to accept the results of the election.

There are less than three weeks left in the campaign, and there are no more guaranteed opportunities for Trump or Clinton to command a huge public audience, as they do at the conventions and the debates (although, they’ll get plenty of attention, of course). Millions of people have already voted. Trump has had a significant advertising deficit, and an even more significant deficit in terms of his turnout operation. He’ll probably spend a significant chunk of the remaining news cycles quarreling over his contention that the election is rigged, and with the numerous women who have accused him of sexual assault. He doesn’t have an obvious — or even a not-so-obvious — path to the presidency.

So we’re left to argue about the probability of an unforeseen event, or a significant polling error. It’s perhaps significant that almost no matter what news has occurred, and there’s been a lot of it — terrorist attacks, mass shootings, foreign crises, her email scandal, the Wikileaks dump, her Sept. 11 health scare — Clinton has almost always led Trump in the polls, although there have certainly been times when the election was close. What if her State Department emails are sitting on one of Julian Assange’s servers? That would be interesting, I suppose. But there are also October (or November) surprises that could work against Trump: more accusations from women, more damaging videotapes, further leaking of his tax records.

The other possibility is a massive polling failure. There aren’t really any direct precedents for a candidate coming back from this far down to win an American presidential election, although you can make a few loose analogies. Harry Truman’s comeback over Thomas Dewey in 1948 almost works as a comparison, but Truman wasn’t coming from as far behind as Trump is, and there was much less polling in 1948. Ronald Reagan had a significant late surge against Jimmy Carter in 1980, but he was ahead beforehand — and the surge came in large part because of a debate that occurred just one week before the election, whose impact was too late to be fully reflected in the polls. If Trump was going to have a Reaganesque surge, in other words, it probably would have started with a commanding performance in last night’s debate — and not another loss.

Brexit? Even that comparison doesn’t really work. The final polls showed a toss-up between the United Kingdom leaving the European Union or remaining in it, and “leave” eventually won by 4 points. If the polls were biased against Trump by that much in this election, he’d still lose, by a margin approximating the one by which Mitt Romney lost to President Obama four years ago. The primaries? They’re a reminder that one ought to be humble when making predictions. But the polls pegged Trump just fine — in fact, slightly overestimating his performance in many early states such as Iowa.

That’s not to say that a polling miss is impossible. Our polls-only model still gives Trump a 14 percent chance and our polls-plus forecast a 17 percent chance, although that’s before accounting for any impact of last night’s debate or some of the other circumstances I’ve described. Presidential elections are rare events, rare enough that we don’t really know what the tail ends of the probability distribution look like, and it’s prudent to make somewhat conservative assumptions under those conditions. It’s possible, also, that the polls are significantly underestimating Clinton rather than Trump — perhaps a combination of Trump’s lack of a ground game and his voters’ feeling despondent because he says the election is rigged will yield a double-digit loss.

If Clinton wins by a clear margin, it will help to resolve a longstanding debate among political scientists and historians, since it will suggest that campaigns and candidates do matter and that elections aren’t always determined by economic conditions, which would predict a much closer outcome than the one we’re likely to see. Furthermore, Clinton’s win will have come by rather conventional means. Her big surges in the polls came following the conventions and the debates. She got the largest convention bounce of any candidate since at least 2000, and she won the debates by a clearer margin than any previous candidate in the six elections in which there were three debates that CNN polled.

Clinton (vs. Trump) 2016 +35 +23 +13 +71
Obama (vs. McCain) 2008 +13 +24 +27 +64
Clinton (vs. Bush) 1992 +14 +42 +0 +56
Kerry (vs. Bush) 2004 +16 +2 +13 +31
Romney (vs. Obama) 2012 +42 -7 -8 +27
Bush (vs. Gore) 2000 -7 +13 -2 +4
Clinton was the most dominant debater to date

Elections without three debates (or without post-debate polls for all three debates) are not shown.

Source: CNN, USA Today/Gallup Polls

There was nothing flashy about Clinton’s performance at either the convention or the debates. She was just prepared, steady and tactically smart — such as goading Trump into feuds with the family of Khizr Khan, or Alicia Machado. Trump might seem like an easy opponent to take down, and he certainly hasn’t helped himself. But as Trump himself would probably point out, 16 Republicans failed to do so. We won’t know for sure for another 19 days, but Clinton may have finished him off last night.

FiveThirtyEight: Who won the third presidential debate?

Nate Silver founded and was the editor in chief of FiveThirtyEight.