One of the major argument against Sanders is that his proposals are too good to be true, that they would not pass the senate and congress.
However, the same people are against Trump for the reason that his hardline policies will be disasters, such as using the nuclear weapons ("his hand is at the button"), and the "wall", etc., etc.
So those same people (aka Clinton supporters) think Sanders can't realize his plans, but Trump can.
Apparently they think Trump will have the full support from the Senate and Congress, while Sanders doesn't.