I write this on Thursday morning, Zagreb-time. Biden stands at 264 electoral votes (Fox News count), with four states left to count. At some point in the next 24 hours, Biden will be declared the President-elect.

Much of America is asking itself this morning: How did Biden make up so much ground when the mail-in votes were counted?

I can think of three factors.

1. Democrats have traditionally used vote-by-mail more than Republicans have, so you would expect there to be more Dem votes than GOP. You can see proof of this in the counts of vote-by-mail results in states that allowed those votes to be counted before Election Day. Vote-by-mail results in those states heavily favored Biden, and Dems outnumbered Republicans when party affiliation was announced.
2. For months, Trump has been telling everyone who would listen that they shouldn’t use vote-by-mail. He and his supporters should not then claim be shocked that most of the votes-by-mail would be for Biden.
3. COVID. Trump voters are more likely to think that COVID is a hoax, and to go to rallies or vote in person. Biden voters are more likely to be afraid of COVID, and to choose voting-by-mail instead of voting in person.

Add those three factors together, and you should expect that Biden will dominate the votes-by-mail.

Why did Pennsylvania, Wisconsin, etc., NOT count the votes-by-mail before Election Day? Answer: the secretaries of state and/or governors of those states attempted to get the GOP-controlled legislatures in those states to change the laws so as to provide for early counting. The GOP-legislatures, likely under orders from the White House, refused.