A good point from Petey:
Specifically, I know that Dem primaries are basically proportional. In other words, if you win 30% of every Dem primary, you'll end up with around 30% of Dem delegates.
But I have a vague impression that GOP delegate selection rules include Winner Take All primaries. Or in other words, you could win 30% of every GOP primary and end up with 50% of delegates.
Right. Democrats normally allocate delegates proportionally to all candidates who pass a 15 percent threshold, whereas Republicans work like the electoral college where if you win a plurality of the state's voters, you win the state. Thus, it's possible in principle to secure the nomination with a fairly small proportion of the total vote. That only works, of course, if more than one viable opponent stays in the race, but it could work.