The difference of two numbers can simultaneously have both positive and negative sign, depending on the position of the viewer (left or right side of the aisle).I don't think this actually works in the real universe, but in the bubble universe of Washington D.C. it appears to hold.
Edit: I confess that I suck at physics. This probably is a manifestation of Einstein's special relativity rather than Heisenberg's uncertainty principle. First, it's not the act of observing the difference from a particular vantage point that makes it positive or negative; it's the location of the observer (and/or the observer's velocity toward the extreme of his/her party's ideology). Second, there is no uncertainty involved: one party is absolute, irrefutably certain the difference is positive; and the other party is equally certain the difference is negative. To admit a grain of doubt is to cause your party's bubble universe to collapse noisily and, worse still, to concede the possibility the other guys are right (this once).