Two Guesses

[Nym90]

You certainly raise an interesting point about realignments; it's more about the culture and the views of the two parties themselves rather than one party becoming dominant. We may be seeing the beginnings of a progressive realignment with the Republicans nominating McCain, who it could be argued is the most liberal Republican nominee since Ford. Realignments result in both parties moving in one direction, such that the partisan balance doesn't necessarily change, but yet one side's arguments pretty much become accepted as true and both sides evolve to the new climate.

Other evidence of a progressive realignment could be the emerging consensus on the acceptance of global warming/climate change and for the need for alternative energy sources to replace petroleum, growing favorability towards national health care and increased taxes for the wealthy, increasing skepticism towards unrestricted free trade, etc. Also one could cite increasing acceptance/tolerance of homosexuals and a trend towards more support for gay marriage and/or civil unions, though that's a more long term trend.