In case you're watching the Puerto Rico primaries at home, here's a quick primer in how the 20 delegates are allocated.
If a candidate gets a majorty of all votes, he gets all 20 delegates.
If not, the 20 delegates are allocated proportionally among all candidates with 15 percent or more -- that will be either two candidates, three candidates, or four candidates.
So, depending on how the other candidates fare, dropping from 50.1 percent to 49.9 percent could cost Romney 10 delegates -- and even more than that, in effect, because it would allow Santorum to gain delegates.
If Romney doesn't get 50 percent, the most he could get is 14 delegates (if Paul and Gingrich, say, got 14 percent each, leaving Santorum with about 20 percent).
So, the second-best result for Romney, after winning a majority of delegates, would be a first-place finish with Santorum down below 15 percent, thus winning no delegates.
Again, the key here is this: how many delegates will Romney gain on Santorum here, or vice-versa.