That’s literally just politics.
And see, that’s the problem people complain about. They are doing bad politics by ganging up to the candidate that’s gaining support from the people who are looking for someone different. Those people end up in Trump’s camp when the Ds can only offer more corporate elites.
I think it would be better if the voters decided to gang up on Bernie and compromise with Biden over the others. Or the other candidates didn’t run at all. This way they just made a mockery of the election process.
Edit: seriously, think about it. Why are you running at all if you’re fine with some other centrist taking over eventually? Just to keep up with the illusion of choice? Stay out of the race and let people see they didn’t have a choice in the first place.