But 60 seats can be a mixed blessing. With 60 seats, the Democrats will have no excuses, no one else to blame, any time they can’t hold their big caucus together. Their most independent, unpredictable members will enjoy massive power — not just Specter but also Lieberman and Ben Nelson of Nebraska, another centrist.

And Specter has always been hard to please. He’ll still be the 60th vote on every issue, just as he was on the stimulus bill — the one who always has a special request before he can say yes. Reid will sometimes wonder whether this was such a good deal.

At the same time, a 60-vote majority can turn into an invitation to abandon bipartisanship. Obama set out to be a post-partisan president, crafting bills that could attract Republican support, but the stimulus plan turned into an exercise in polarization. If healthcare legislation becomes a Democrats-only affair, that will be the end of Obama’s commitment to bringing along both sides of the aisle.