Fiorina will probably be the first. She has little support and no money and can’t even point to being a current or former member of an elected body to fall back on.
Next comes the ones with no support and little money, like Graham, Kasich, and Pataki.
Carson’s just like the above, but crazy, so he’ll probably stick it in a little longer. Plus, his agenda’s different. Even he doesn’t think he has a shot at winning, he’s there to sell books and get a gig on Fox news, so he’ll stick around a little longer until he gets embarassed, which is probably around Iowa.
Then we have a few former/current governors who think they have a shot owing to their past record of winning, but will face reality as soon as the actual votes start coming in. Jindal will go, then Perry and Huckabee, then Christie after he sees how badly he’s doing.
By the time of the first votes, guys like Santorum, who stuck around, will be looking at the crowded field knowing they can’t win but trying to be kingmakers. He got like #2 last time, but with more choices in the primaries, I don’t think he’ll make it that far. He’s also in it to raise his profile and pretend to be relevant, but 2012 was his peak and he’s not reaching that again. He’ll drop out and throw his support behind the most virulent fundie still left.
That leaves mostly the serious guys, the ones who are currently high-profile and flush with money and/or elected officials. Of the rest, its hard to tell who will crazy themselves too much and flame out first, but I’m guessing a combination of Paul, Cruz, and Rubio will stick around for more than a handful of primaries until they are mathematically eliminated. I never bought Rubio’s threat to win, he’s too moderate, too foreign, and made mistakes on immigration to ever capture the primary, but he’s a better candidate than Santorum. Maybe in 2020.
I’m sure that Bush will make it to the final 3, but I’m just not sure if he’s going to be battling against Trump and Walker or one of the other guys above. I don’t really see Walker as anything more than a regional lacky. Sure, he’s got Koch money and his anti-union stance is beloved by conservatives, but he’s also got the least name recognition of probably the top 6 (with Cruz/Paul/Rubio rounding that out). Money helps, but not if, by this time, your opponents get to define who you are.
Trump I just can’t figure out. He might be seriously crazy and believe his own lies, in which case he’ll stay in as long as possible, making racist remarks and getting the base all fired up, or he can do it for a few months, not even make it to Iowa, and decide that he’ll drop out and focus on his business. He’s either all in or trolling everyone, including the RNC, I just don’t know