A well known principle of the branch of Psychology known as Psychophysics, Weber's Law, "states that the ratio of the increment threshold to the background intensity is a constant. So when you are in a noisy environment you must shout to be heard while a whisper works in a quiet room." In its quantitative form, di/I = k, where I is the intensity of some stimulus, di is the amount of change in the stimulus necessary for the change to be noticed, and k is a constant. So if the stimulus value is I=4 and the change is di=1, then the constant is k = 1/4 = 0.25. Now if the stimulus value were quintupled to 20, (and k=0.25), then the di needed for a change to be noticed is 5 (5/20 = .25). The implication is that a change of only 1 unit in a background of 20 units would not be enough to be noticed.
What does that have to do with politics? When there are just 4 possible candidates and another declares his or her candidacy, it is noticeable and the media is all over it like flies around a horse's behind. But now if the pool is expanded to 20 candidates, one more or less does not matter and is barely noticeable. Why? Because of Weber's Law.
Rick Santorum, in announcing his candidacy kind of late, may just have fallen victim to Weber's Law. Think of Santorum as an insufficient stimulus. Couldn't happen to a nicer GOPlin.