Matthew Yglesias imagines a counterfactual in which the Democrats enjoyed more legislative success during Obama’s first year in office:

If congress had committed itself to expeditiously enacting administration proposals on stimulus, on health care, on climate, and on financial regulatory reform the economy would be in better shape, Obama would be more popular, and congressional Democrats would be more popular.

Boosting the economy via a larger stimulus would likely have improved Obama’s standing, and thereby the Democrats’, but I’m skeptical that passing the other measures would have increased their approval ratings. It seems more likely that a series of liberal legislative victories would accelerate the expected shift in public opinion toward a preference for smaller government, and thereby reduce the popularity of Obama and the Democrats in the short to medium term.