Why isn't this a null set?

Democrats who believe that the GOP southern strategy is a defining moment that discredits the entire movement ∩ Democrats who think that the Clintons are very deliberately playing up Obama's race in order to drive white voters into Hillary's camp ∩ Democrats who will vote for Hillary Clinton in November once she secures the nomination.

This article available online at:

http://www.theatlantic.com/business/archive/2008/01/why-isn-apos-t-this-a-null-set/2600/