This was one that the Republicans really should have won.

Given the weak economy, American voters were open to firing President Obama. In Europe, in similar circumstances, one government after another lost re-election. And, at the beginning of this year, it looked as if the Republicans might win control of the United States Senate as well.

Yet it wasn’t the Democrats who won so much as the Republicans who lost — at a most basic level, because of demography. A coalition of aging white men is a recipe for failure in a nation that increasingly looks like a rainbow.

Schadenfreude may excuse Democrats’ smiles for a few days, but these trends portend a potential disaster not just for the Republican Party but for the health of our political system. America needs a plausible center-right opposition party to hold Obama’s feet to the fire, not just a collection of Tea Party cranks.

So liberals as well as conservatives should be rooting for the Republican Party to feel sufficiently shaken that it shifts to the center. One hopeful sign is that political parties usually care more about winning than about purism. Thus the Democratic Party embraced the pragmatic center-left Bill Clinton in 1992 after three consecutive losses in presidential elections.