The special election of a new Senator in Massachusetts offers the rare opportunity for a clear tipping point in national politics. If the Republican candidate, Scott Brown, wins in the historically overwhelming liberal state, it will create strategically political momentum for change. It is already surprising that Brown is so close in the polls. A Brown win would likely lead directly to a revolutionary mid-term election, which will be less about political parties than the proper role of American government.
Tipping points are fairly rare occurrences when a number of small changes can create a large shift. One key to creating tipping points is momentum. Strategically, momentum isn't a matter of just winning but of a surprise success (7.0 Creating Momentum). If the Democrat wins in Massachusetts, it will create no momentum because it is expected. However, if a Republican wins, it will force everyone to completely reconsider the shape of political landscape (1.2.1 Competitive Landscapes).
If you want to help create a tipping point, you can donate to support Scott here.