Donald Trump is about 550 delegates short of earning the 1,237 delegates necessary to win the Republican nomination.

If he keeps winning at the pace he has so far, he will probably take around 60 percent of the remaining 900 or so pledged delegates, reaching his target. But his path is deceptively tenuous, and it might not take much to knock him off.

It all hangs on whether Mr. Trump can continue to fend off Ted Cruz in states where Mr. Trump is relatively weak. He barely did it in Missouri (with an official result still pending) and North Carolina last Tuesday, when Mr. Cruz showed unexpected strength.

The Trump Base

Mr. Trump can cover about half the distance of what he needs simply by sweeping the states where he’s expected to fare well. The voting patterns and demographics of the remaining states look pretty good for him. He has an added advantage in that the delegate rules over the second half of the primary season make it easier for him to earn lopsided delegate tallies in many states.