Well, I wish I wasn't writing this answer, but it seems to be true - the support that Trump is getting from huge proportions of the population is very worrying but also very real. Americans seem to be dumb enough to think Trump is actually a valid candidate - and if he is elected, then God help us all.
While there are a large number of people who are against Donald Trump, there are also a large number of people who support him. Bernie has the support of many millenials, but Trump has the support of many conservatives, especially extreme ones. Trump's method of scare tactics is negative, but sucessful.
Don't get me wrong, I would much rather have Sanders win, but Trump has tons of supporters, and millions of people who are probably willing to vote for him. The sad thing is that a lot of these people might vote for him based on name recognition alone, which is terrifying to think about.
No one in the GOP takes Donald Trump seriously. He is a thorn in their side and they cannot figure out how to get rid of him. He has managed to offend Muslims, women, the handicapped, Latinos, and blacks. All of whom are a major segments of the electorate. He will not be able to win without having them vote for him. Also, a great deal of Republicans will either stay home on Election Day or vote for the other candidate. There's no way he can beat Bernie Sanders.
Donald Trump will not be able to defeat Bernie Sanders in a presidential election. It is unlikely that the United States will ever see that particular match-up, but if it did, Sanders would be the victor. Trump's views are too polarizing and inflammatory for a majority of voters to endorse.