I've said it many times already, but the reality of his supporters (and his lack of any actual positions, and thus mirroring theirs) is the coming to power of white power nationalist fascists. They will threaten with violence and jail every minority, every dissenter, and press to ethnically cleanse the US. Many historians will make their career 100 years from now writing on the bloody end of the United States if these skinheads win.
It'd be bad, don't get me wrong. Severe economic depression, an end to America's place on the world stage, mass deportations, likely some blackshirt action as well. It would be a terrible four years, certainly our worst since the Civil War (or, if we're lucky, Hoover). But barring a war, the amount of actual lasting damage he can do to the Republic is limited. Even if he gets a couple Supreme Court picks, the Court isn't going to kowtow to him and would stop the most unconstitutional actions. And I doubt he gets Congress to play along if he tries to just ignore the Supreme Court altogether à la Jackson.
Of course, note the phrase, "barring a war"; if he gets us in a shooting war because he gets pissy, or lashes out, or wants to distract himself from domestic concerns, all bets are off.
