As promised in my last post, here are the rationales for the probabilities I gave for the Republican nomination. On Jeb Bush, I'm just going along with prevailing opinion. He has a big lead in fundraising and endorsements, and most people who are supposed to know seem think that this is important. He's been slipping in the polls in the last week, and if I were to give numbers now he would be lower. Beyond that point, my thinking is that the main issue is the domestic economy. On that, almost all candidates are trying to show how conservative they are--ie, how opposed to taxes and spending. There are some differences in the exact promises: income tax rates of 17%, 14%, 12%, 10%, tax returns that fit on one page, tax returns that fit on a postcard..... but it's hard to keep them straight and I doubt that many voters take them seriously.
The result is that there are only a few candidates who stand out. One is Donald Trump, who has obvious liabilities. Another is John Kasich, who suggested that government spending can sometimes accomplish good things, and that when it does he'd support it. He's the only one (except maybe Trump) who is trying to appeal to voters who have conservative inclinations but aren't ideologically committed. There are a lot of voters like that, so I could put Kasich higher, but he starts with a disadvantage in money and name recognition. Also, many activists and donors regard this position as heresy, so I think if he gets close there will be a strong "anyone but Kasich" movement. Finally, there's Marco Rubio, who is Hispanic. "Diversity" has become a mainstream value, and I think that given the choice between candidates who are reliably conservative and about equally qualified, a lot of Republican voters would pick one who's black or Hispanic. Rubio doesn't have the negatives of Ben Carson (no political experience) and Ted Cruz (not likable enough), so he's the one who can benefit from that.
That leaves the 12.5% for a late entrant. Candidates who came in late have a very poor track record in recent primaries (Rick Perry, Fred Thompson, Wesley Clark), but since a lot of Republicans think that 2016 is a good opportunity and no one in a large field has caught fire (except Trump), it seems like there's an unusually good opportunity.