Democrats allocate most of their delegates proportionately; candidates are awarded a cut of the delegate pie based on their percentage of the vote. It is possible to lose a state and still get a majority of the delegates, and it is likely that the losing candidate will still get a substantial share of the delegates. …

The Republicans’ delegate selection rules are different. In 8 of the 21 Republican contests, the winner gets the delegates — no dividing up the spoils. What that means is that it is going to be easy for a candidate to build up a big delegate lead on Tuesday night and, combined with winning some big states, credibly declare himself the party’s presumptive nominee. That is precisely what Mr. McCain is looking to do.

-Adam Nagourney, "As 24 States Vote, a Grab for Delegates, and an Edge," New York Times