I have to admit that with each passing week, I am less certain that Donald Trump will not be the G.O.P. nominee. But I won't believe it until Nate Silver over at fivethirtyeight.com says it's possible.
What he and his staff are saying now is that it's down to a three-man race with Trump, Cruz and Rubio. The way the primaries fall, Trump and Rubio will do better in the early primaries and if Rubio's going to win, he will come from behind in the later ones. This article charts how well each man must do (roughly) on each primary date to be on a path to the number of delegates necessary to win the nomination. If you're really interested in this race, read what they say. It'll tell you what to watch for.
Also: If you're really concerned it will be Trump, take a look at this article on the various candidates' favorability. Trump, as the headline says, is really unpopular with general election voters. And while you're at it, read Jonathan Chait on the Trump campaign's estimation of his strengths. Yow.