It's not a coincidence that the two people who received the least scrutiny and attacks (Kasich and Sanders) polled the best. And the reason they received so little scrutiny/attacks is because everyone knew they had no chance at winning.
For a month or two Ben Carson had the highest favorability rating and GE numbers due to the fact that he was mostly ignored and was just seen as the nice friendly likable black doctor. Then he completely collapsed under the slightest bit of scrutiny. Sanders' image and poll numbers would take a hit if he was the nominee or looked like the presumptive nominee. The only question (which none of us will ever know) is how big the hit would've been.
Considering the fact that according to polls American voters were more willing to vote for a gay or an atheist candidate rather than someone who calls himself a socialist, it would have been a pretty big hit.
