1. He has insulted Hispanics, women, Muslims, Catholics (by attacking the pope), POW’s. There are not that many groups left to fashion a winning coalition.
2. If he wins the nomination, wait until the opposition unloads. You think what John Oliver did skewered The Donald? You ain’t seen nothing yet.
3. By this time in every presidential nominating contest save one (GOP-1976), a consensus had formed in the late stage primaries around one candidate. That has not happened here. Since only about 20% of registered Republicans vote in the primaries, and since Trump is polling about 40% of the vote, in effect he is garnering support from a mere 8% of all Republican voters. If he cannot “close the deal” with the mainstream Republican voter, he could be on course to post a record low for a major party candidate in the general election (Goldwater and McGovern set the modern mark for futility with 38% of the popular vote.)
4. With the exception of 1948, when a nominee faced a split in his own party, or a decided indifference from the mainstream elements of his party, he went down to defeat. Again, see McGovern (’72) and Goldwater (’64); see also Humphrey (’68), Ford (’76), Carter (’80), Bush (’92), and Dole (’96).
5. The average voter would relish the opportunity to “get even” with someone blessed with an abundance of riches. Think about what the 47% said at the polls to Mitt Romney. Heck, think about what you would love to say to your boss. This is the chance for the many to tell one of the well-to-do, “You’re fired!”
6. His head won’t fit in 1600 Pennsylvania Avenue. Seriously. Americans like the ideal of Cincinnatus grudgingly answering the call to duty. We know politicians have egos, but most of them (Hillary, Bernie, Cruz, Kasich) are able to convey passion for others and not merely self-absorption.