Lexicus
Deity
You're comparing the lower rungs of political spectrum. I'm talking about the highest. In Europe, if you have ambitions, you usually ride with party closest to your ideology. In US, if you have the funds, you can try going in as independent, try a third party, or take the path of least resistance and hitch a ride in one of main parties. But if you differ from the ideology of the bosses, sooner or later they'll bar you from advancing. That's what happened to Sanders in 2016 primaries, if you don't remember. He was just too different, so they had plans to rig the primaries against him, as revealed by leaked emails.
LOL, what? In Europe, the party "bosses" have much more direct and indirect power over candidates for office. In fact, in the US the parties are basically nonexistent at the institutional level, so much so that even talking about party "bosses" is anachronistic.