No.
For example in 2008, Edwards had a sizable chunk of the vote but his candidacy wasn't really helping or hurting either Clinton or Obama. Some said he took a good chunk of the "not Clinton" vote, but you can see after he dropped out that wasn't really the case as his supporters ended up about evenly split.
In 2012, Romney has proven himself to have a ceiling. It seems a majority of Republican voters want someone else, and as a known quantity Romney doesn't have the ability to change that. During the race we've seen the "not Romney" vote stay fairly consistent, with its focus swinging wildly on various candidates over the course of last year. Now it is just Gingrich and Santorum left (I leave Paul with his own faction). Gingrich and Santorum are both positioned as the "true" conservative choice versus Romney, who isn't trusted by conservatives. I think it is fair to say if either dropped out, a substantial majority would go to whatever "not Romney" candidate is left, thus giving that person a plurality (I think Santorum is more strongly positioned to do this).
2008 doesn't prove me wrong either because Edwards wasn't running to the right or left of anyone. He was just there.