Trump is a sleazebag for sure, but he's a sleazebag with a message that resonates with Voters.
The strength of Sanders is that his message also resonates with Voters. He also has near impeccable integrity.
But the weakness of Sanders is his messages are vague and unclear on how to actually achieve his goals. Sanders wants workers to have more money and people to have better health care. Great, wonderful, fantastic! How does it actually work? Can't say we're going to demonize corporations, demonize banks, demonize "the rich", they'll pay all sorts of new and higher taxes, and that makes the rest of the world return to harmony! Doesn't work quite like that.
He has "near impeccable integrity" because he doesn't do anything of substance.