In the early years of this country, candidates from the same party frequently ran against each other. Thomas Jefferson and Aaron Burr were from the same party in the 1800 election. Each won or lost electoral votes on his own.
The election of 1800 is the reason for the 12th Amendment. Jefferson and Burr each received the same number of votes—even though Jefferson was running for President and Burr for VP. The Founders did not account for political parties in their original plan for electing President.
Burr saw this as a chance to win the Presidency, but was thwarted in this by Hamilton’s urging Representatives to vote for Jefferson rather than Burr. Perhaps one reason why VP Burr shot and killed Hamilton in a duel.
In any case, Biden’s dropping out before the Electoral vote would put democrat Electors into a bit of a bind. Vote for Harris? Who wanted her to be President? Vote for democrat-approved replacement for Biden? In any case there would likely be chaos. Don’t know how the public would react to an unelected last-minute Biden replacement.
But a massive Trump victory makes all of these scenarios moot.