Your question is phrased in the singular, but the answer is very much in the plural. Romney got over half in a couple of dozen states, at least, although the higher percentages were mostly in the states voting later. In the end, Romney got 51.5% of the total primary popular vote, so it's not fair to say he won by a mere plurality. Lots of numbers here.
The rules were actually fairer this time around. Most states were not winner-take-all, so there was room for less popular candidates to collect delegates. For instance, I voted for Newt in Massachusetts on March 6. When I checked the results and found he got only 4.7%, I realized I should have voted for Santorum or Paul, each of whom came much closer to the 15% threshold to take a delegate away from Mittens, who got almost 72%.
Here's a map showing how the counties sorted out:
Orange is Romney's color. The darker the shade, the higher the percentage. E.g. check out Utah (LOL!).