The obvious point is that the Republicans would successfully destroy a new candidate with this little time before an election. It takes YEARS to defend against lies, or truths, and define a candidate. It would take about two weeks of news cycles to suppress the D vote.
How would kamala Harris be attacked in ways she has not already been attacked, she's the vice president of the United States. Are they gonna call her a loony California liberal even though she was a prosecutor? certainly no republican has accused her of that before
I mean, they call literally EVERY Democrat a Marxist commie extreme leftist, even if you're an old style conservative KKK democrat - you can go moderate, you can pander, they're still gonna call you a commie socialist