With her polling lead slipping, Hillary Clinton still has Pennsylvania as a firewall — for now.

David Rothschild, an economist at Microsoft Research who runs PredictWise, an online forecasting model that relies on betting markets, explained the primacy of Pennsylvania for Mrs. Clinton’s election chances during an interview this week.

He also discussed the potential importance of the ground game, the perceived failure of betting markets in predicting “Brexit” and the ethics of live forecasting on Election Day, among other topics. Here’s the lightly edited text of our email exchange.