Not probable, but we can't rule it out. The field is too large, and the math involving delegates is too uncertain.
I'm cheering for a very unlikely situation: a brokered convention.
One odd side effect of our campaign finance rules in the wake of Citizens United is that all a candidate needs is a few high-dollar donors, and he's set for the long haul financially. The days where a candidate performs poorly in the early states, only to drop out as donations dry-up? Those days are gone. With enough cash banked early, multiple candidates can hang around a good deal longer into the process, dividing-up delegates and preventing the electorate from coalescing around one non-Trump candidate.
By my estimate, at the very least we'll see Cruz, Bush, Walker, Rubio, and Trump all be able to stick around well into March. We're still a looooong ways off from actual votes being cast, so this is purely academic.. but if Trump is topping-out around 25-30%, with the others all dividing-up the remaining vote, he could stick around and win some winner-take-all states' delegates and some proportionally-distributed states' delegates, giving him enough delegates to make waves at the convention.
I cheer for a brokered convention because it'd leave the GOP nominee broke and bruised, and there would be sore feelings from the faction of voters whose candidates didn't end-up on the ticket. That.. and it'd be pretty cool to witness some convention drama at least once in my lifetime. Save for the occasional hum-dinger of a convention speech (Clinton in 2012, Eastwood in 2012, Richards in 1988, etc), these pre-planned, hyper-coordinated events are pretty boring. 😋
