Conservatives: Many of Trump's ideas are left wing and at odds with what conservatives usually believe. Yet there are those like Anne Coulter who embrace his position on immigration so much that they'd be fine if "Trump performs abortions in the White House." For them, immigration trumps everything else. (What? What? Okay. Pun intended.)
Moderates: While Trump may be extreme on some issues he's in the middle on others. He isn't a rigid conservative like Ted Cruz.
Independents: Trump puts himself above party and many independents dislike the political parties. One of the perplexing things to me has been why Trump chose to run as a Republican instead of an independent. Yes, it's easier to win with a party behind you and a two person race, but he is in a a no man's land where he's moving to the right on some issues and not on others. Trump supporters like his independence but you have to give up some of that to run as a Republican. This is a problem Bernie Sanders is having too. He's always been an independent, but he's running as a Democrat. Like Trump he's not a good fit for a political party and like Trump him being independent of a political party should be part of his appeal. Both want to say they can't be bought but if you're going to ask for a party's nomination you have to bend somewhat to the party.
Democrats: At one time White middle class blue collar voters were the backbone of the Democratic party. The Democrats have been making them feel unwelcome and that they favor minorities, gays,and everybody else instead of them. Yet they are an uncomfortable fit for the anti-union Republican party they've always seen as "the party of the rich." Some have become Republicans, but others have remained Democrats. Until now. Donald Trump has done extremely well in New England, a place with the highest percentage of White Democrats. Trump is running a non-ideological campaign and isn't presenting a slew of policies Democrats oppose. Sure, immigration might be a hang up but there are Democrats who don't agree with the Democratic party on that issue.
Minorities: Some might think that minorities hate Trump and many do. But you can be Black or even Hispanic and want to get tough on immigration. Arnold Schwarzenegger had a lot of Black and Hispanic appeal due to his celebrity and celebrity is part of Trump's appeal. He's anti-establishment, and that he says what he thinks. You don't have to be White to appreciate these things. I'm not saying the Trump is going to do well with Blacks or Hispanics but I'd guess that Trump will do better with Blacks than other Republicans would and won't bleed as many Hispanics as people think he will.
Trump's support is so difficult to peg down because, for the most part, he isn't running on policy and isn't running left wing or right wing. Since that's how most politicians run Trump breaks the mold. I've heard from Republicans who insist they won't support him, even if it means handing the Presidency to a Democrat. They think that Trump will do very poorly as a result. While it's true he polls worse than other Republicans he likely also brings in voters who wouldn't vote for a Republican otherwise. I don't think the latter group is larger than the former group, but I've learned that I shouldn't underestimate Trump.