Edit 9 Nov 2016: I was wrong. To avoid publication bias, I will leave this post up. It will teach people not to trust my political judgement.

Commenting on Trump is fashionable lately, so let me jump on the bandwagon. Probably these points have all been made before.
It seems the people most against Trump are the Democrat supporters, which suggests they are ignoring strategic voting. A famous voting example is that if A is preferred to B by the majority, B to C and C to A by different majorities (1/3 of people prefer A to B to C, 1/3 prefer B to C to A and 1/3 C to A to B), then with naive voters the order of votes matters. If first the A vs B vote is held and the winner goes against C, then A wins against B, after which C wins against A. But if B and C are voted first, then B wins, after which it loses to A.
One possibility is that Trump is preferred to other Republican candidates, who in turn are preferred to Democrats, who are preferred to Trump. In this case the Democrats should strategically support Trump against other Republicans and be happy that the primary vote between Republicans happens before the general election, not after. A conspiracy theorist might even suspect collusion between Trump and the Democrats or at least secret Democrat support for Trump to split the Republicans, like in the plot of All the King’s Men.
What if the polls show swing voters to favour Trump over the Democrats? Who people claim to support in elections is not necessarily who they actually support (the Bradley effect). Voters may claim to support Trump as a joke, or actually favour him now, but reconsider closer to elections. Putting Trump in power is like a dangerous adventure – there is a thrill at the possibility and many (claim to) want to do it when it is in the distant future. When the opportunity actually arrives, it may look too scary and people may get cold feet.
In the end, if Trump actually becomes President, he has antagonized many Republicans. There is a good chance of a bypartisan effort to block all his initiatives. The crazy and illegal things Trump has promised to do are just election promises – politicians break those all the time. Even reasonable promises are broken – radical ones are even more likely to be ignored. If Trump tries to do the unconstitutional lunacies, both parties have an incentive to impeach him. The enemy of my enemy is my friend, so taking down Trump may be just the thing to make Republicans and Democrats strange bedfellows and narrow the political polarization in the US.