Democrats are of course not in fact powerless at all. But they have adopted an agenda that only a supermajority could pass (if that, even a supermajority couldn’t pass cap and trade), and with every indication of public opposition have only intensified their determination to pursue it, putting themselves on the wrong side of independent voters while persuading themselves that people would come around because this health care bill is something liberals have wanted for three generations.* They have made it impossible for themselves to change course without a massive loss of face and of political capital.
It's not often that Mother Jones agrees with the National Review. But I have to say that Levin is mostly right (at least in this particular section of his post). The Democrats aren't powerless. They still have large majorities in both houses of Congress—they could pursue passing a revised bill through the reconciliation process (unlikely) or have the House pass the Senate's version of health care reform (also unlikely, but slightly less so). And Levin is right that the Democrats did adopt an agenda only a supermajority could pass.
That's because real reform of anything—unburdened by the limitations that reconciliation rules impose—really does require 60 votes in the Senate to overcome the inevitable filibuster. That affects Republicans, too. Bush's main reform effort—the partial privatisation of Social Security—would almost certainly have failed to overcome a Democratic filibuster. Tax cuts and wars aren't reform. The Democrats wanted to actually do something, and they nearly succeeded. They still could. After all, as Levin writes, it's impossible for them to change course without "massive loss of face and political capital." It'll be interesting to see what they do.
*My one objection to the above block quote would be to point out that this health care bill is not what "liberals have wanted for three generations." It's far too conservative for that to be true.