Whatever you think of Trump, Krugman is (Ironically) even more of a mess, just not as hot. Here, he labels Trump "conservative"-- probably for political motives. But as this post notes, Trump has more in common with Democrats.
All of this misses a big underlying issue: the strange need to explain the complex American political "spectrum" along two dimensions (only) and with simplistic terms like "liberal" and "conservative".
Trump is one more "outsider" candidate who is attracting populists. (Check out this attempt at thorough analysis of the catalysts for Trump's emergence. See also: Pat Buchanan. Bernie Sanders is the outsider on the elite/polished, statist-oriented side.) Outsiders have a lot of room to play these days, given that we've had two historically bad presidents in a row. Trump is drawing "Tea Party" voters within the GOP and the "conservative" (and often racist) Democrats I met during the 2008 primary.
When Trump fades/disappears in the next eight weeks, those voters will look for somewhere else to go and-- depending on the particulars of their preferences-- will move to Cruz, Paul, and Clinton.