Nate Silver, who interprets political polls the way Jascha Heifetz played the violin, says Donald Trump ain't as popular as some people — chief among them Donald Trump — say. Yeah, 20% or so of Republicans say they want him as their nominee but you can't win the nomination with 20%…and Trump's negatives suggest he'll have a hard time driving that 20% much higher.
I would also add that it's still a long, long time until people have to cast a real vote. I think a lot of them are enjoying the show he's putting on and not listening too much (or caring) about what he'd actually do in office. Let's see where he is when it comes time to get serious.
Also, Silver doesn't mention this but I have it on excellent authority that Rick Perry, Carly Fiorina, Rick Santorum, Bobby Jindal and Lindsey Graham all now have lower favorability ratings than Dr. Walter J. Palmer. Perry and Fiorina are, however, polling slightly ahead of Bill Cosby.