Democrats and the Republican operatives raising money from Rachel Maddow viewers all say that Donald Trump is a unique threat and menace to this country. If Joe Biden believes that, he will choose a running mate who is most likely to help him win the election. In that case, he should absolutely not pick Kamala Harris.
Biden is currently leading Trump in national polls and swing-state polls largely by staying out of the spotlight and letting Trump’s negatives drag him down. It’s an old rule in polling that “Generic Democrat” or “Generic Republican” generally outpolls every specific Democrat or Republican. That’s especially true when you’re running against an unpopular incumbent prone to public meltdowns. Biden, hiding in his basement, is as close to a generic Democrat as one can get.