They dumped Biden because the Republicans harped and harped on Biden's mental capability and he performed poorly in the debate.Maybe. But they're going to win. Hell, they actually played it beautifully. Installed the alternative candidate at the last possible second to avoid any chance of significant scrutiny. Just put a Not Trump t-shirt on someone and count the votes. It just happened to be Harris.
The Republicans are the ones that created this scenario.
Surprise, surprise, Trump and the Republican uber-race was not prepared to help old-man Trump to run against anyone other than old-man Biden. They just don't know how to do that.
Trump already sucked but now he still thinks he's running against Biden. It won't work. Trump still sucks and it is more apparent than ever when he uses the same useless tactics against Harris.