In a Saturday email, Biden campaign manager Jennifer O’Malley Dillon urges supporters not to be complacent and says the race is much closer than the public polls indicate, “We are not ahead by double digits. Those are inflated national public polling numbers.” She argues that they’re only leading by three points in some key battleground states, and emphasizes that polls can be off, as they were four years ago, and that Trump can still come from behind.

Yet, at one point she relents in her alarmism: “I know we’re going to win Arizona.”

Arizona is a key state. Here’s an example that demonstrates the point. If Biden flips just Wisconsin, Michigan and Arizona and all the other results remain the same from 2016, that would result in a 269-269 tie in the Electoral College, and the whole thing would go to the House of Representatives. Of course, if Biden were also to win Maine’s 2nd congressional district, that would be sufficient to give him a 270-268 victory.

It’s not clear that Biden would win in the House of Representatives. Each state will vote based on how many members of each party are in their delegation, and right now the Republicans have more state majorities than the Democrats despite being in a rather deep minority overall. The exact ratio will change after the election, but Biden really can’t rely on winning a tied race.

Still, winning Arizona could get him to a tie even if he loses Pennsylvania, Florida and North Carolina, where he leads, and states like Ohio, Georgia, and Iowa where the race is extremely tight. If Biden has Arizona locked up, he’s in pretty solid shape.

O’Malley Dillon’s advice is both prudent and wise. No one should be complacent. But it’s clear that Trump is facing a very difficult task.