The Dems have the right to nominate whoever the hell they want. That's not the point, and as usual, because that's who he is, Paul's doing his best to obfuscate the facts.
Joe Biden, his handlers, the DC Dems and the media LIED to their constituents. Everyone with half a brain knew that Biden only had half a brain. But, because, Dem voters consume a very narrow source of news (cheap fakes! stuttering!) they apparently weren't aware of Biden's cognitive decline. Those same collaborators conspired to subvert democracy for their voters and the general public. They voted for Biden on false pretenses.