Decrying the party’s “activist wing,” Barack Obama last week issued a characteristic warning to Democrats not to stray too far to the left:

Even as we push the envelope and we are bold in our vision we also have to be rooted in reality. The average American doesn’t think we have to completely tear down the system and remake it.

Obama’s intervention — an obvious jab at those who want Democrats to embrace an ambitious agenda centering policies like a Green New Deal and Medicare for All — was fittingly cynical for a man who began his ascent in national politics by preaching transformation and governed as a consummate Beltway insider. Nonetheless, in his appeal to realism, the former president touched on something vital about how liberals today understand their politics and role in the world: namely, as guardians of a rational, incrementally minded center synonymous with objective reality over and against the bonfire of enthusiasms that pervade on left and right.