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In this election, polls failed to predict that President-elect Donald Trump was winning key battleground states, but a start-up in San Francisco had spotted that advantage well in advance. It wasn't because of an enthusiasm gap - Republicans turning out, and Democrats staying at home. Instead, the start-up's data pointed to a big crossover effect - Democrats voting for Trump in droves. NPR's Aarti Shahani reports.
AARTI SHAHANI, BYLINE: The start-up Brigade built an app and asked a simple question - who are you going to vote for? - pretty much like boots-on-the-ground organizers do in the physical world, though CEO Matt Mahan explains one big difference is that people share their pledge.
MATT MAHAN: It's a little bit of a change from what most people are used to. People don't go around in the offline world broadcasting who they're voting for. They may share it with friends or family, but I think many people feel that it's a fairly private decision.
SHAHANI: Most people aren't wearing their button for, like, the 18 months leading up to the election.
MAHAN: That's right.
SHAHANI: The app seemed to work. Brigade has gotten nearly 200,000 unique users - not Twitter bots or trolls, but citizens the start-up has cross-checked with voter registration records to verify identity. And when Mahan looked at the pledges, he sees data to back up that popular hashtag, #NeverTrump, among Republican voters.
MAHAN: So for registered voters on Brigade, 95 percent of Republicans pledged to vote for Donald Trump and only 2.2 percent pledged to vote for Hillary Clinton.
SHAHANI: That's roughly what you'd expect. Here's where it gets fascinating.
MAHAN: On the Democratic side, we saw something entirely different. Of our universe of registered voters, only 55 percent of registered Democrats pledged to vote for Hillary Clinton.
SHAHANI: Repeat - 55 percent, just a little more than half. I ask for a clarification.
And is this while Bernie was still in the race?
MAHAN: This is during the general election.
SHAHANI: So it's not the Bernie Sanders effect. It's Trump.
MAHAN: Forty percent of registered Democrats pledged to vote for Donald Trump.
SHAHANI: I'm sorry. How many?
MAHAN: Forty percent.
SHAHANI: Brigade saw this pattern back in September, but they didn't trust it because their user base skews conservative. Even though the company was started by liberals in San Francisco, it went viral in Republican circles. Once the election happened, brigade realized, oh, yeah, we were on to something. Let's take the swing state North Carolina. Brigade's Aleks Mistratov.
ALEKS MISTRATOV: Donald Trump beat the final projection by 4.5 percent. And it was also a state where about 25 percent more than our baseline of Democrats pledged to vote for Donald Trump.
SHAHANI: That is, when he compared North Carolina to the overall baseline of Democrats crossing over on the app, he saw it happening even more at a far higher rate there. And it happened in other swing states, like Pennsylvania. These results break political science as we know it. For months, we've been telling the story of how Trump has upended the Republican Party. Turns out, there were early signs that he's upending the Democratic Party, too, and possibly with the voters you'd least expect, like women.
MISTRATOV: Interestingly, one of the data points we just pulled in the last hour indicated that a higher percentage of these registered Democrats crossing over to vote for Trump were women.
SHAHANI: Huh (ph).
MISTRATOV: Yeah, it's interesting.
SHAHANI: Mahan says researchers, pollsters should drill into the states where Donald Trump outperformed the polls and look closely at what registered Democrats did and why. Aarti Shahani, NPR News, San Francisco.