Seth Godin linked to this from his site and used a theory I'd heard before, one that's much simpler than the one in the article.
You're buying a drink and have a choice between a small and medium drink, which would you purchase?
Now, there's three options. Small, medium and large. Which do you buy?
With two choices, you gravitate towards the smaller. When there's three, you will buy the medium.
This article (washingtonpost.com) suggests that the two Democratic and two Republican front-runners would be best-served to figure out who the #3 competitor is and make friends with them. People, processing quickly and irrationally, when unable to choose between #'s 1 and 2 will look at #3. Their opinion of #3 will influence their choice between the top two candidates. Very interesting.