It's harder to estimate how many mail-in ballots will be rejected when election officials begin voting on Tuesday. Not only does Pennsylvania reject mail-in ballots if voters have not signed or correctly dated the affidavits on the outside of the return envelopes, or have not placed the ballots in inner secret envelopes, but it also prohibits counties from “acquiring” or mailing-in ballots in advance open election day. Some counties identify mail-in ballots with deficient affidavits or missing secrecy envelopes before Election Day without opening them, but some leave the task to Election Day. For this reason, we cannot determine the true status of absentee voting in some counties.

In total, our panel of available statewide data shows 5,524 defective mail-in ballots per day as of Monday – 2,258 are missing signatures, 2,471 are missing data or misdated and 795 have no secrecy envelopes. The data also shows that about 4,400 voters used state-specific guidelines to “cure” or fix their problematic mail-in ballots.

In counties where we believe election officials returned verified mail ballots based on affidavits or secrecy issues, about 0.8% of mail ballots were ever recorded as defective. If this is true, we would expect a little less than 20,000 defective mail-in ballots statewide – although the defect rate could be higher in other counties.

Voters who returned faulty mail-in ballots and still need to repair them can vote on Election Day, usually through provisional ballots. Those provisional ballots will be counted, according to the state Supreme Court's ruling last week.

However, at least in 2022, relatively few voters made use of this option. Meanwhile, as noted above, an estimated 35,000 could cast provisional ballots after failing to return their mail-in ballots at all.

Looking forward, we expect there will be a lot of discussion on election night and in the days afterward about provisional ballots cast by people who requested absentee ballots. There may be another lawsuit in the future reconsidering whether to count provisional ballots cast by people who previously returned defective mail-in ballots. But when it comes to Pennsylvania's margin of victory, it's important to remember that many more provisional ballots will be cast by people who don't return their mail-in ballots at all.