Trump supporters will think the election was rigged and respond accordingly. This will likely mean protests and maybe even riots at what they see is a stolen election and injustice.

A lot of Democrat supporters on the other hand genuinely believe it will be the end of US democracy if Trump wins. Even saying it will be the last election the US ever has.

This belief will spur a much more violent reaction than simply thinking it was stolen. Imagine if you genuinely believed the US will never have another election and will turn into an authoritarian dictatorship. Wouldn't you go to any lengths necessary to prevent that?

Republicans will be angry at a stolen election. Democrats will be afraid for their life. The latter is always going to react more desperately than the former.