Terry Madonna from Franklin and Marshall College tells Jim Gardner that Democrats outnumber Republicans in early and mail-in voting by a 3 to 1 ratio.
On the flip side, Republicans are expected to outnumber Democrats in day-of voting, and those are the votes that get counted first.
The early and mail-in ballots cannot be counted in Pennsylvania until Tuesday, and they must be counted by Friday.
WATCH: Pa.'s Early Numbers and Final Results May Be Very Different
Because of the time it takes to open and process those ballots, it is likely that Democrats will be underrepresented in the day-of count, and over-represented in the mail-in count.
Not only could Pennsylvania be undecided on Tuesday night, it could be an example of what some people are calling the "red mirage," an impression that Donald Trump is winning, when in fact, many Democrats haven't even had their votes counted.