Biden’s position would simply be a whole lot safer if one of two things were true: If either the polling in Pennsylvania were like that in Wisconsin and Michigan, where he has a larger lead … or if another state such as Florida were also polling more like Pennsylvania to give Biden a clear Plan B. But neither of those things are true. The gap between Pennsylvania and Wisconsin and Michigan has actually grown in the waning days of the election. And no state has emerged out of the pack of Arizona, North Carolina, Florida and Georgia to be Biden’s clear Plan B (Biden is forecasted to win each state by between 1 and 3 points). Arizona is probably Biden’s best bet in this group, but winning it would also require him to win either Nebraska’s 2nd Congressional District or Maine’s 2nd Congressional District to break a 269-269 Electoral College tie; he’s favored in both districts but they aren’t sure things.
Moreover, because polling errors are somewhat correlated from state to state, if Biden loses Pennsylvania, he would no longer be a favorite in states such as Florida and Georgia — where he’s narrowly ahead now — because it would be a sign that Trump had outperformed his polls again. Of course, this only goes so far: In 2016, Trump massively outperformed his polls in the Midwest, but there wasn’t much of a polling error in Arizona. Still, losing Pennsylvania would take Biden from favorite to underdog.