Nate Silver writes that in order to become the GOP nominee for the 2016 election, Trump must jump the “Six Stage of Doom”:

Stage 1: Free-for-all

When it happens: This is the stage we’re in now; it will continue through the next couple of months.
Potential threat to Trump: Increased attention to other GOP candidates.

Stage 2: Heightened scrutiny

When it happens: Mid-November or thereabouts, as voters up their level of attention to the campaign
Potential threat to Trump: Polling support doesn’t translate to likely, more-informed voters.

Stage 3: Iowa and New Hampshire

When it happens: Feb. 1 and Feb. 9, based on the provisional calendar.
Potential threat to Trump: Middling performance in one or both states, either in an absolute sense or relative to polls.

Stage 4: Winnowing

When it happens: mid-February through mid-March
Potential threat to Trump: Other candidates drop out, and remaining ones surpass Trump.

Stage 5: Delegate accumulation

When it happens: mid-March through final primaries in June
Potential threats to Trump: Poor organization in caucus states, poor understanding of delegate rules, no support from superdelegates.

Stage 6: Endgame

When it happens: June through Republican National Convention, July 18-21
Potential threat to Trump: The Republican Party does everything in its power to deny him the nomination.

The full article is a fun and informed read, but Silver got to his 2% by assuming that Trump has a 50% chance of being knocked out at each stage, meaning, for example, that Trump only has a 6.25% chance of making it through the “Winnowing” stage.