There is a special election in Pennsylvania's 18th district Tuesday that could provide a clue as to what we'll see in November. Managing editor of political site Sabato's Crystal Ball, Kyle Kondik, says the party that holds the White House historically pays a penalty down the ballot.

The election comes because Republican Congressman Tim Murphy, who has held the seat since 2003, resigned. Kondik says Congressman Democrat Conor Lamb is culturally a good fit for the district and has out-funded Republican rival Rick Saccone.

Sabato's Crystal Ball rates the race as a true toss-up, which is somewhat shocking, because the district has always been red. Even if the Democrat loses, Kondik says, the fact there is even a tight race shows just how tough November could be for Republicans.

Sabato's Crystal Ball incorrectly predicted, like so many others, that Hillary Clinton would win the presidential election in 2016. Kondik says a lot of pollsters got caught up in Donald Trump's "strange candidacy," but traditional, political science models still have a track record of accurately predicting elections.