John Cassidy outlines it:
For his campaign to take a real hit, I think his share of the vote would have to fall well below thirty-five per cent, and his lead would have to fall to ten points, or less. Even then, that might not be too big a problem if Paul were to be the second place finisher—Paul isn’t a serious threat for the nomination. What would be truly calamitous for Romney would be for his vote share to drop below thirty per cent and for Huntsman to come in second place, with, say, twenty one per cent of the vote.
Via Andrew Sullivan, who writes, “I stick to my view that anything near 30 percent for Romney looks very weak.”