He didn't say anything like that. All he said is that people won't think his views are extreme once Santorum is out of the race.
There is no logic behind your claim. No matter how "extreme" a candidate's view are, they aren't a reflection on the other candidates in the race unless, 1) other candidates do in fact share those views; or 2) as in Romney's case, they try to argue that they are even more "severe" in their beliefs. Ron Paul has run on a series of absurdities yet neither Romney, Santorum nor Gingrich were ever been smeared with the "libertarian" brush.