If this was truly his plan, step one would not be “give up a seat in Congress and leave the vacancy’s appointment to someone who has endorsed my opponent.”
I get the attractiveness of the proposal, but the reality is that while the vast majority of Americans believe that Congress is a problem, they also don’t believe that *their* member of congress is the problem.
There are lots of reasons to vote for Sanders. “Because there will be more people like Sanders in Congress” is not one of them. If Sanders wins, there will be precisely one less person in Congress like him.