This thread is silly and ignorant. Trump and his cons started this rumor. Any Democrat has a few days left to file and qualify to run against Harris. But none have and most of the likely have thrown support behind Harris. Who, within a few hours, already has the delegate votes to win at the convention.
Biden is not the dynamic carnival barker screamer of lies that Trump is. While Biden helped make the US economy the strongest major economy, More jobs were created, the biggest successful accomplishment in a terrible Congress, and the largest infrastructural bill passed since the Interstate highway system, and the most support for alternative energy in history etc. He has been a great President. But the constant lies and attacks by R's and their Putin-loving leader are believed since Trump is a showman. Trump is full and energy and lies, and Biden is the rational person that gets things done when he can without the majority in the House. But he is not a showman, criminal, or fraudster, and scews up words and people's names worse than Biden.
However unlike the glory of Trump who only cares about winning even by fraud and lies,Biden is more concerned about people and saving the US from a dictator/Criminal, Fraudster, Sexual abuser etc of Trump who refused to accept a fair election and supported an voilent insurrection to try and hang Pence who refused to refuse the will of the people. Of course Trump lost by an overwhelming total vote count but we have the silly electorial college system which is why Trump won last time and was leading in the polls in the six battle ground electoral states.
When he displayed aging issues (also reported by staff in private meetings) and the polls, groups of youth, blacks etc that formed his winning coalition showed they were not comfortable with him being able to win against Trump, he did what was best for the country and tossed his support to Harris.
Within about 24 hours, money and support for Harris poured in with a record amount of funding and enthusiasm. Harris is the best choice to save the US from such a dictator only for the wealthy vs the average citizen and saving the democracy as we know it.