Over at Swampland, Karen Tumulty distinguishes a "filibuster-proof majority," which is what FDR had, from a Senate where one party or another controlled a theoretically filibuster-proof number of seats, which is what Carter had. Specter's defection, she suggests, is akin to FDR's situation rather than Carter's.
But I'm not sure that's right. FDR, after all, couldn't pass health care reform, and so didn't even try. The presence of the Dixiecrats led him to almost totally exempt African Americans from Social Security. His court-packing scheme was a flop. There is, in other words, no such thing as a majority that renders the president's agenda safe from congressional opposition. The simple fact that a majority could be filibuster-proof in general doesn't mean it is filibuster-proof on discrete pieces of legislation. The question, rather, is simply how much gets passed. FDR got a lot more passed than Carter. But it's a continuum. Arlen Specter's sudden admiration for progressive principles has not rendered single payer a likely outcome or given Obama a free hand to pursue a harsh carbon tax. Rather, it has turned a potential vote for Obama's priorities into a likely vote for Obama's priorities. That's an improvement, but it's not clear that it's a gamechanger.