WHAT THE HECK IS GOING ON?
There are, obviously, a few possible answers to that question. Trump would say he is tapping into what the American people really want: a brashness and boldness that makes us (America) the good guys and everybody else the bad guys. He tells it like it is and people respond. That the political establishment and media are so surprised at his rapid rise speaks only to how out of touch those groups really are.
I think there may be a psychological element to add to the technical explanation offered by Philip. Put simply: I do think Trump's name ID plays a role in what's happening in polls, but I don't think that's all that's at work. Everyone knew Trump's name a month ago but he wasn't at or near the top of the polls then. So what's happened? What's led to such a stunning reversal in how he's perceived among Republican voters? And what makes someone (or 17 percent of someones) say, "Yes, I think Donald Trump is the best Republican candidate for president"?
Here's my working theory: Trump's rise has very little to do with "Donald Trump" and very much to do with what Trump represents at the moment to voters.
What is that? Someone who doesn't sound or, frankly, look like other politicians. Trump's willingness to say impolitic things (Mexican/rapists etc.) actually bolsters his appeal among a certain segment of the Republican electorate, not solely because they happen to agree with his hard-line views on immigration, but because they see him as being willing to say uncomfortable things and shake up the system.
Trump's purposeful(?) positioning as the ultimate anti-politician is perfectly suited to the consequence-free mindset that dominates among likely Republican primary voters at the moment. The election is a million miles away to the average voter, so what's the harm in telling a pollster you like Trump right now? You almost certainly won't stick with him when it's time to actually vote — just 41 percent of Republicans view Trump as a "serious" candidate for president in a new Gallup poll — but he is a great vehicle to send a message to the political establishment that you're sick of them.
Think of it this way: You're single. You're in your 20s. You start dating a bad boy/girl who makes you laugh/kind of scares you/your parents hate with the white-hot passion of 1,000 suns. You are not planning to marry this person. In fact, if he/she did happen to pop the question — they wouldn't because they are a bad boy/girl and "don't like to be tied down" — you would almost certainly say no. You are going out with this person because it's fun and it's different and you are tired of the squares — is that a word people still use? — you've dated up to this point. Sure, you'll probably settle down with one of those squares (I am just going with it) but not just yet.
The tendency in politics — particularly among the journalistic and consultant classes — is to overthink things. And so it is with Trump. But I think his appeal may actually be very, very simple: He's different — and different, at least right now, is intriguing for Republicans. My strong belief is that as the election gets closer and Trump goes from colorful oddball to "Hey, wait a minute, he believes what?!?" his support will slip — probably considerably.
For now, Trump represents a middle finger from a segment of Republican voters directed at the GOP establishment. Which seems about right.