Mark Meredith and Michael Morse for NBC News:

Overall, about 2.2 million Pennsylvanians have had approved mail ballot applications…. [R]oughly 390,000 mail ballots are outstanding.

If Pennsylvania’s 2024 election follows the same trend we observed in 2022, then about 150,000 more mail ballots will arrive in time to be counted, via either the mail or drop boxes…. [Further, in] 2022, about a third of voters who requested but did not return their mail ballots ultimately voted in person. Taking 2022 as a guide, we would expect around 90,000 to vote in person, with about 35,000 using provisional ballots….

It is more difficult to estimate how many mail ballots will be rejected once election officials start to canvass them Tuesday [because of missing data]…. Overall, our panel of the available statewide data by day show 5,524 deficient mail ballots as of Monday — 2,258 are missing signatures, 2,471 are missing dates or are misdated, and 795 lack secrecy sleeves. The data also shows that about 4,400 voters have taken advantage of county-specific polices to “cure,” or fix, their problematic mail ballots.

In counties where we expect election officials to have checked mail ballots returned to date for any affidavit or secrecy issues, about 0.8% of mail ballots have ever been recorded as deficient. If that holds true, we’d expect a little fewer than 20,000 deficient mail ballots statewide — but the deficiency rate may be higher in other counties.

Voters who have returned deficient mail ballots and have yet to fix them can vote on Election Day, typically by provisional ballot. Those provisional ballots will be counted, according to the state Supreme Court’s decision last week. However, at least in 2022, relatively few voters took advantage of that option.