I think he did, but from that field, he might have been the best of the bunch. The alternatives after Iowa and NH were McCain or Huck. Of those three, I went with Romney, too.
You guys are getting your facts wrong. DeMint endorsed Romney at the START of the primary season, on Jan 9, 2007. At that time, the primary field was still wide open and it was anybody's to win, with the expected frontrunners to be Fred Thompson, Rudy Giuliani, and Mitt Romney, not McCain. South Carolina is one of the early primary states and votes long before Super Tuesday -- Fred Thompson, Duncan Hunter, and everyone else was still actively in the race when South Carolina voted. Furthermore, Romneycare had already become law when DeMint issued his endorsement, and said at the time he was happy with what Mitt Romney had done with health care in Mass. (he has since rescinded those claims)
DeMint's endorsement of Romney in Jan. '07 was a statement that DeMint felt Romney was the best choice in a crowded field of many Republicans. To spin it was a last minute "stop McCain" endorsement when all the conservatives had dropped out is simply not true.