This is a pretty long, meta blog post. You've been warned. But try to stay with me here. Matthew Yglesias has a good observation here that I can't figure out a way to discuss without, alas, quoting in its entirety:

Here’s an intriguing swathe of Ryan Lizza’s profile of Rahm Emanuel:

“They have never worked the legislative process,” Emanuel said of critics like the Times columnist Paul Krugman, who argued that Obama’s concessions to Senate Republicans—in particular, the tax cuts, which will do little to stimulate the economy—produced a package that wasn’t large enough to respond to the magnitude of the recession. “How many bills has he passed?” […] “Now, my view is that Krugman as an economist is not wrong. But in the art of the possible, of the deal, he is wrong. He couldn’t get his legislation.”

Whether or not you think Emanuel is right about the legislative politics, it seems to significant for the White House Chief of Staff to concede that Krugman is correct about the economics and the legislation President Obama signed into law may, in virtue of its concessions to conservatives, be too small to rescue the economy.

I also always find this particular form of ping-pong to be a big odd:

  1. Practical Politician A offers Proposal X.
  2. Outside Commenter B says that X is too moderate on the merits.
  3. Practical Politician A angrily retorts that better legislation on the merits would have been impossible to secure.

I think the right way to understand the (1)/(2) dynamic here is that the criticism in step (2) makes it easier to secure the passage of legislation. If you propose something, and every single progressive in all the land immediately lauds it as the greatest bill ever written, then your legislation is now an extreme left proposal and it’s doomed. If you’re going to make concessions to political reality then you need to weather a bit of criticism from your left—that’s what establishes the proposal as moderate and sensible. Things like “some liberal economists such as Paul Krugman say the proposal is too small” is a helpful piece of context-setting that prevents the proposal from appearing too radical.

Ok. I agree that it's necessary for Democratic politicians to have critics on their left. One of the points I made in my book (now out in paperback!) is that most people, even political journalists, do not pay much attention to or understand the merits of policy, especially on economic legislation. They base their judgment on heuristics, one of the most important being what policy position represents "the center." This means that efforts to capture the center by adopting the most popular position are often self-defeating-- if Republicans take an extreme position, and Democrats try to move to the center, then the center just becomes the middle point between the extreme Republican position and the centrist Democratic position. It pays to capture the center on easily-understood issue, but on complex issues it makes much more sense to stake out a more purist stance and have critics to your left.

Therefore, if you want a reasonable legislative outcome, then almost by definition Democrats need to have critics on their left who are unreasonable, either in their political methods or their policy goals. This was one of the main points of my article about the netroots two years ago -- the netroots are both unreasonable (at least some of the time) and necessary for sensible moderate policies to prevail.

Back to Yglesias. I agree with his basic analysis. I disagree that Rahm Emanuel was reacting to Krugman "angrily" -- by Emanuel's standards or even by normal human standards. But my main disagreement is that pushback by Democratic politicians is also a necessary element of th process. You need prominent outside commentators to take positions to the left of the Democrats, and you need the politicians to push back against the commentators. That's one of the ways they establish their own centrism. The theme of my book is that the political center has been redefined rightward for the last thirty years. Ideas that were once considered moderate are now liberal. You can't reestablish the center without also reestablishing the distinction between the center and the left.

--Jonathan Chait