Well, two political sciency thoughts relating to this.
First, electoral stability can be misleading. We've got used to Americans not turning out to vote, and 'Red' and 'Blue' states remaining solid, with just a couple of 'battleground' states deciding the result of the election. Trouble is, every now and again there are big shifts in electoral support - Obama has taken Florida and Ohio comfortably, and even Virginia by a comfortable margin. Possibly North Carolina too. Now this may be a blip, but it fits with Paul Krugman's claim in 'The Conscience of a Liberal' that a clear trend towards Democratic dominance has emerged over the last decade or so, relating to demographic change, registration of new immigrant votes, and the decline of race as a polarizing issue in an increasingly non-WASP America. This election has probably consacrated that shift, entrenching the Dems in power for a generation. Maybe. The point is that electoral change may be glacial but becomes visible rather abruptly (partly because our empirical observations are few and far apart - when is a trend a trend rather than a blip?).
The second point is that unequal societies eventually generate a reaction against unfair politics and institutions. The US (like the UK) has presided over a vast increase in inequality over the last 30 years, which has been accepted in part because of a lack of clear alternatives to free market ideology, and in part because of a failure to mobilize the losers from this process. In 2007-8, both these reasons have lapsed. The free market binge has created an economic disaster that even neoliberals can't bring themselves to deny, and Obama's campaign has used the energy of a new generation of voters and new fund-raising technology to mobilize the disenfranchised. The key here is mobilization - Obama has a broad base, and is less beholden to special interests than, say, Clinton. This, and his skin colour, are strong reasons for marginalized social groups to be convinced that voting is not a total waste of time. In other words, the progressive movement must mobilize the base, and truly progressive politics cannot work if you have to make pacts with monied interests. That is the limit of the New Labour strategy - let's see if Brown learns the lesson.