Donald Trump’s somewhat surprising win has forced many political analysts to wonder: Were we wrong all along in thinking Hillary Clinton had the upper hand, or was late-breaking movement to Trump part of the reason why polling averages missed his upset Electoral College victory? There’s certainly evidence that the polls underestimated Trump’s support in crucial Midwest states. But the latest wave of the Institute for the Study of Citizens and Politics panel survey that my University of Pennsylvania colleague Diana Mutz and I have been overseeing is now complete, and it provides new evidence that voters did shift to Trump in the final weeks of the campaign, too.

Panel surveys differ from other polls in that they re-interview the same people repeatedly, allowing us to see how specific Americans’ attitudes shift over time. They thus help us sidestep the problem that some groups of people might be more likely to take polls when their candidate is thought to be doing well or receiving favorable press coverage. Our October 2016 wave was conducted with nationally sampled adults over age 26 between Oct. 14 and Oct. 24, meaning that it ended soon after the third Clinton-Trump debate. At the time, Clinton was riding high in the polls — and 43 percent of our panelists in that wave expressed support for Clinton, as opposed to 36 percent for Trump. By way of benchmarking, this same group of panelists had gone for President Obama over Mitt Romney 46 percent to 39 percent in October 2012.

At first glance, it might seem as if Clinton in October 2016 was in roughly the same position as Obama was in October 2012, at least with respect to the distribution of votes nationally: Both enjoyed margins of 7 percentage points among exactly the same group of people. But there were critical differences, even beyond the fact that the geographic distribution of support is crucial in making one candidate president. First, the number of undecided respondents in 2016 was 21 percent, significantly outpacing the 15 percent we saw in 2012. Second, our 2016 survey ended on Oct. 24, leaving two full weeks before the Nov. 8 election for people’s minds to change. There was still a lot of time on the clock.

And while most people’s support remained the same, the changes we did observe were consequential. Consider the table below, showing panelists’ support in the October 2016 poll versus their support in the post-election poll, which took place from Nov. 28 to Dec. 7. Eighty-nine percent of the 1,075 American adults reported the same preference in both waves, whether it was for Clinton (38.0 percent), Trump (35.2 percent) or neither (15.8 percent). But among those who did move, Trump had the advantage. While no one moved from Trump to Clinton, 0.9 percent of our respondents moved from Clinton to Trump. Although that 0.9 percent isn’t a lot, those changes are especially influential, since they simultaneously reduce Clinton’s tally and add to Trump’s. If there were a comparable swing in the national electorate, 1.2 million votes would move to Trump.

clinton post-election trump post-election neither post-election CLINTON IN OCTOBER 38.0% 0.9% 3.1% TRUMP IN OCTOBER 0.0% 35.2% 1.7% NEITHER IN OCTOBER 2.3% 3.1% 15.8% Change in candidate support pre- and post-election, 2016 Panel survey conducted among 1,075 adults from Oct. 14-24 and from Nov. 28 to Dec. 7 Source: Institute for the Study of Citizens and Politics panel survey

Trump also outpaced Clinton among people who were previously undecided or third-party backers, with 3.1 percent of respondents moving from those categories to Trump while just 2.3 percent did the same for Clinton. Clinton also saw 3.1 percent of her October supporters defecting to third-party candidates or becoming undecided. Trump lost just 1.7 percent.

In all, Trump picked up 4.0 percentage points among people who hadn’t been with him in mid-October, and shed just 1.7 percentage points for a net gain of 2.3 points. Clinton picked up a smaller fraction — 2.3 points — and shed 4.0 points for a net loss of 1.7 points. That’s certainly consistent with Trump gaining steam in the race’s final weeks. Seeing as the 2016 election was held on the latest possible day given the mandate to hold it on the first Tuesday after the first Monday in November, we might just add the 2016 leap year to the ever-growing list of reasons why Trump prevailed.

To put these shifts in context, let’s look back at the same panelists’ replies before and after the 2012 election. Then, our pre-election wave ran from Oct. 19 to 29, while our post-election wave began on Nov. 14 and ran through Jan. 29.

obama post-election romney post-election neither post-election Obama

in October 46.7% 0.3% 0.8% Romney

in October 0.9% 40.0% 0.4% Neither

in October 3.5% 3.4% 3.9% Change in candidate support pre- and post-election, 2012 Panel survey conducted among 1,075 adults from Oct. 19 to 29, 2012, and from Nov. 14, 2012, through Jan. 29, 2013 Source: Institute for the Study of Citizens and Politics panel survey

Despite the fact that many post-election interviews were held later in December 2012 and even into January 2013, that race showed considerable stability from the October numbers: 90.6 percent of respondents had the same preferences, with 0.9 percent moving from Romney to Obama and 0.3 percent moving the other way. The movement out of backing third-party candidates or being undecided was relatively symmetric, with 3.5 percent of all respondents moving from the undecided/other category to Obama, and 3.4 percent moving to Romney.

Of course, it’s possible that what we are seeing in 2016 is movement after the election, since our post-election survey didn’t start until late November. Maybe people claimed to have backed Trump only after seeing that he won. But the stability we see in 2012 suggests that respondents aren’t likely to change their responses so soon after Election Day — in 2012, the winning candidate was viewed more favorably than the winning candidate in 2016, and the post-election interviews stretched well in January.

As to what moved these Americans in the final weeks of the campaign, the panel has little to say. The timing of James Comey’s letter to Congress — released on Friday, Oct. 28 — makes it one potential explanation. When making sense of campaigns, people often search for overarching narratives, and Comey’s letter provides a ready-made story. No less a political observer than Bill Clinton recently explained his wife’s loss by pointing to Comey’s letter.

Still, we shouldn’t discount the possibility that voters might have gravitated to Trump anyhow. Research has long suggested that over the course of a campaign, partisans come home to their party’s candidate. Between mid-October and our post-election wave, Trump picked up almost 4 percentage points from people who had backed Romney four years before, suggesting that Republican identifiers were doing just that. Trump’s media coverage in the final two weeks was markedly more positive than it had been during the prior weeks, and it’s possible that shift in coverage was just the opening some Republicans and Republican-leaning voters needed to get behind Trump.

In key respects, the 2016 election represented a decisive break with recent historical patterns. But in an election of surprises, Trump’s victory might have come from small electoral shifts in the campaign’s final weeks that were surprisingly unsurprising.





Nate Silver and Michael Lewis on the 2016 election