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Clinton’s Post-Debate Bounce

September 29, 2016

Polling shows that Americans who watched the first 2016 presidential debate on Monday thought, by landslide margins, that Hillary Clinton bested Donald Trump. Trump couldn’t have chosen a less opportune time to choke – the debate was watched by more Americans than any previous presidential debate. So the question isn’t so much whether the debate will help Clinton’s campaign, it’s really how much it will help.

The best gauge of the short-term impact of discrete events like the debate is the “now-cast” measure of election outcome probabilities on Nate Silver’s FiveThirtyEight.com. A day’s now-cast gives the percentage chance that each candidate would have to win the election if the election were held on that day – it is not a prediction how the vote will go on Election Day.

The now-cast is much more sophisticated than indexes like the much-noted polling average maintained by RealClearPolitics.com – first, because the now-cast is based on computer modeling and statistical analysis much more sophisticated than RCP’s simple averaging, and second, because the now-cast is based on state-by-state analysis, not just national polling.

The period after the two national conventions was a disaster for Trump. On July 28, the last day of the Democratic convention, the now-cast said that Trump’s chances of winning the election were 52.9 percent. But after the conventions, Clinton took off, peaking on August 6, when 538 rated her 96.4 percent likely to win the election if it were held that day. Clinton more than doubled her odds of winning in nine days, from less than 50-50 to the next thing to a dead certainty. From the August 6 peak, Clinton went into a steady decline, falling to a 52.1 percent chance to win on September 26, the day of the debate.

Then Clinton whomped Trump in the debate.

In the three days since the debate, as of this writing (538 updates its election prediction indexes as polls come in, often several times a day), Clinton’s now-cast chances of winning the election have risen to 71.9 percent.

Even more important is the now-cast’s state-by-state breakdown. This is important because while Silver’s record for predicting election outcomes is perfect at the national level, it is very nearly perfect in predicting how each of the 50 states and the District of Columbia will vote – not just in presidential elections but also in senatorial elections.

As of now, the now-cast gives one candidate or the other more than 70 percent odds of winning all states except six: Arizona, Florida, Iowa, Nevada, North Carolina, and Ohio. But here’s the thing: counting only states where one candidate has more than a 70 percent chance to win, Clinton would have 272 electoral votes if the election were held today – two more than needed to win. (I’m counting one of Maine’s electoral votes as up for grabs, because Maine awards electoral votes by congressional district, and I’ve seen reporting that Trump is viable in one district.)

In other words, even if Trump won all six states that are in play he would still lose the election. And there is no reason to believe he will win all six – the now-cast awards Florida, Nevada, and North Carolina to Clinton.

The bounce is especially timely because early voting, which tends to favor Democrats, opened in two more states today, including the swing state of Iowa. With each passing day, the extent to which the election has already happened increases, and the number of votes remaining to be cast decreases.

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