right now the two biggest frontrunners for the Democratic primaries are Joe Biden and Bernie Sanders. Why is that?
Joe Biden is a creepy, corrupt, out of touch establishment Democrat. The only reason people like him is because he comes off as a bully who can defeat the bully in power.
Bernie Sanders is basically an independent with a lot of socially progressive views. He's far from the only one though so why do people like him? Because people feel that he would have beat Trump in 2016.
So what happens if one of these two men take the nomination and the Republicans do an about-face and run Mike Pence, a man who is pretty much Donald Trump's polar opposite? at that point the Democrats would have essentially brought a hockey stick to a basketball game.