To be fair, assuming Romney wins the nomination, Newt will endorse Mitt too. With the exception of Paul and very few others, they will all back him. Newt, Santorum, Perry, Palin, etc, etc, will all endorse, support and actively campaign for Romney. Don't fool yourself into thinking otherwise.
To those Newt supporters they see a a compromiser rather than a fighter of principles when they see Santorum you know "well, we didn't get what we really wanted but at least we got SOMETHING done"
I just see Santorum as a terrible nominee because I think he'd lose a general election in a landslide so large it would take our House majority down with it. Romney sucks, but Rick would lose even worse. Is Rick a better conservative than Mitt? I certainly think so. Unfortunately, he would not be a better general election candidate. Santorum simply can't avoid being baited over and over and over on the social issues. By the time the media is done with him (and because Santorum is terribly undisciplined), you'd think the election was about whether to ban rubbers and Victoria's Secret catalogue's.
That's the second reason why I never jumped on the Santorum wagon.
The first was that he thinks Romney is a real conservative.
Yes, perhaps Newt, Palin and all the others will end up endorsing Romney in the end but the end is when it's ONE Republican vs Barack Obama and not one republican against another republican.
If Santorum stepped down tonight I still suspect that he would endorse Mitt Romney over Newt Gingrich again calling Mitt the "real conservative". He did it before and he'd do it again. When asked about it Santorum would brush it off as "oh... that's just politics".
That's why many do not trust Santorum's judgment.