Nate Silver: "Without Pennsylvania, Biden becomes an underdog"

Joe Biden will become "an underdog" if he fails to win Pennsylvania and its 20 electoral votes, FiveThirtyEight founder Nate Silver said on ABC's "This Week" Sunday.

Why it matters: Trump won the battleground by less than a point in 2016, as he narrowly swept swing states in the Rust Belt to secure victory in the Electoral College. FiveThirtyEight's average of polls currently shows Biden with a 5% lead in Pennsylvania.

What he's saying: Silver told "This Week" that if Trump ends up winning the election, "it would come down to Pennsylvania."

• "Pennsylvania has not bumped up to a 7- or 8-point Biden lead like we see in Michigan and Wisconsin. It's 5 points. It's not a big early voting state, so a lot of votes have not yet been cast in Pennsylvania," Silver said.
• "Among the votes that were sent in by mail, there are some provisions about a naked ballot, a security envelope. That could make things more complicated. You could have the courts involved. You have some protests, looting in Philadelphia. There's lots of stuff going on."
• "Maybe a lot of little things add up and Biden loses Pennsylvania by half a point, and then he doesn't quite pull off Arizona or North Carolina. He does have other options. ... But still, without Pennsylvania, then Biden becomes an underdog."

The big picture: FiveThirtyEight gives President Trump a 10% chance of winning the election and Biden a 90% chance.