To be frank, judging a candidate by their supporters is silly in itself. I try to not look at the supporters of candidates (although Trump's confirms to me all kinds of awful) because they generally represent the team aspect of American politics. I remember a lot of Obama supporters justifying voting for him because he was black. Not that voting for him because he's black is an inherently bad thing, but when you're doing it absent his actual policies, that's a problem, as seen in the Obama episode on the Boondocks. American supporters of politicians are almost always fervent and rowdy and illogical. This just represents another broken element of our already cracked political system.
Well yes and no. A candidate and their supporters are distinct people. However,
A) The type of people attracted to a candidate often tell you a lot about their message and goals. Like for the Trump example you provided, that a lot of his supporters seem to be a mixture of poorly educated, racist, xenophobic, primarily white people who feel betrayed by the GOP. This a reflection of his frankly racist and xenophobic campaign that puts personality and abrasiveness over policy substance and that has often attacked GOP gospel.
For Sanders, that fact that his support base is predominantly young, liberal and white (not exclusively) is reflection of a campaign & message that puts economic populism above all, frames things in exciting terms like "revolution" and advocates policies that sound very appealing but are hard or problematic to implement - a prospect that is less relevant if someone is less politically experienced or prone to over-estimate the power of the President (young people).
B) The support base for a candidate should serve as a good intermediary and promoter. If you don't know much about a candidate and want to learn more, if the people you meet are assholes in one way or the other, you're going to be turned off irregardless if that's fair. Like if you go to a strore and the volunteers or employees there are assholes to you, you're going to think less of the store and the store owner.
Or to use a real life example - there was a news story in the SC Primary thread where a Sanders volunteered called a woman to try to sell her on Sanders. At the end, he basically said the woman should support Sanders because he advocated for welfare. Said woman didn't take it well for obvious reasons. Obviously Sanders himself would never be that dumb or rude but now that woman has that negative association because said volunteer was and advocate for Sanders.
