It's the lack of a majority. I won't add the Carville "stupid" because A) I don't like it, and B) we are so used to identity politics as ethnically and/or racially based that it is far from evident. But the more I think about it, the more I'm convinced that the real demographic story was not the primacy of the Latino vote or the Asian vote or even the women's or gay vote. The key is that all these groups (including women who, while not a numerical minority, have been treated as one and so identify and behave as a minority), along with Jews and African-Americans and others, voted and identified as a minority. They didn't necessarily vote for their particular ethnic or racial self-interest; they voted for the party that best responded to minority interests in general.
If I'm right, then one of the two most important pieces of demographic news was the announcement, almost precisely a month before the election, that the U.S. was no longer a Protestant-majority nation. This has less to do with the political proclivities of Protestants, who can be as liberal or reactionary as anyone, and much more to do with the other key demographic factor: More and more cities (following the lead of Los Angeles, which hit this milestone nearly a decade ago) exist without any majority at all.
As groups, minorities and majorities behave differently. The most striking example of this occurred during the Holocaust, when Catholics protected Jews in Protestant Germany and Huguenots did the same in Catholic France. What mattered was not their beliefs, religious or political, but the fact of minority status. Sadly, the reverse can be seen in Israel, where Jews became a majority for the first time in modern times and promptly became as repressive as any Christian majority who repressed us. (Although one cannot and should not minimize the effect of constant, unending, war, it is worth noting that when Isreal was governed by people whose life experience was as a persecuted minority, the electorate was reliably socialist. Now that the electorate is mainly people who grew up as a majority in their own country, it votes right.)
Majorities tend toward repression. Minorities instinctively understand that policies that protect other minorities protect them. The coalition that reelected Obama contained many groups that have often been at odds with each other. There's plenty of tension between African-Americans and Latinos. But when faced with a national election in which neither group (or any other group) can predominate, these differences were set aside. (The only exceptions were those minorities with recent history and memory of Communism -- Cubans and Russians -- and ultra-Orthodox Jews, who essentially live as separatists.)
Look at the red states on the electoral map and you're looking at states in which there are still majorities. It's not surprising that the state where I vote, North Carolina, has swung back and forth; it's on the tipping point. Its gerrymandered districts have been carefully crafted to create as many Protestant, rural, white-dominated hustings as humanly possible. Within a decade the current number of these districts will be demographically impossible.
From the time of Nixon's Southern strategy, the Republican Party has presented itself as the majority's haven. Threatened majorities can be very dangerous things, and the Republicans have relied on their fear. For as long as the country had a strong majority, this was extraordinarily effective. And on a local level, when a district can still have a clear majority, it is effective still (hence the House Republican victory). But given the inexorable shift, it won't be enough for national Republicans to suddenly shift on immigration reform. They would have to stop their entire program of repression -- of immigrants, women, gays, etc. -- to gain minority votes.
In the meantime, minorities are doing what minorities must do: realizing that the only way to protect ourselves is to protect others in the same position. And that's not stupid at all.
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