How comfortable are you assuming that all the Gingrich support would shift to Santorum, or all the Santorum support to Gingrich. I think Gingrich and Santorum may be further apart politically than either one is from Romney.
1/2 of that question is being answered in MS. Santorum is dropping, but Gingrich is gaining by a similar margin.
Polls have shown the bulk of Gingrich’s support going to Santorum, mostly out of sheer hatred of Romney. Santorum’s supporters though aren’t really fond of either and seem to be splitting in favor of Romney, if only, because they are tired of fighting what many see as the inevitable.