Not a complete stranglehold, but a near-complete one. Both of these candidates had to jump through the two major U.S. parties' hoops to have a chance at being elected (and one of them was very effectively stopped by his party's gatekeepers).
Sanders didn't play to win and dug his own grave. In one of the Democratic debates: "The American people are sick and tired of hearing about your damn emails!" If I was a Sanders donor, I would have been very upset at that moment.
I am a Sanders donor, and I wasn't upset by it at all. (He was saying they should move on to more important topics).
The point of political campaign is to win, not to take the high ground. He took money from millions of people and flushed it down the toilet with that line.