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.
This sounds right to me, but I wonder if it's really true? Let's turn it around. When Rush Limbaugh criticized George Bush's immigration plan, did that convince liberals that maybe Bush's position wasn't so bad after all? Maybe it did! But I'm not so sure about that.
(At least in the short term. Constant kvetching can certainly change the center of gravity of public opinion over periods of years or decades. But that's a different thing.)
Anyway, it seems like there ought to be some clever way to test this theory. In general, does criticism from the extreme left or right help a bill's prospects with moderates? How might we figure this out? Any ideas?