I would say that only Romney absolutely can not beat Obama, although there are three others still runing who will not get large parts of the base.
If I believed Romney could definitely beat Obama and that the others didn’t stand a snowball’s chance in hell, I might pragmatically hold my nose and vote for Romney. But I’m firmly convinced that Romney is actually the least electable nominee and that Obama would soundly defeat him in the general. Romney needs to be resisted at all costs if an Obama reelection is to be avoided.