When Barack Obama ran for president in 2008, he understood, without quite saying it, that there had been no highly successful Democratic president in decades.

Bill Clinton made the country a better place, but his biggest legislative plans failed and he was beset by scandal. John F. Kennedy, though popular in retrospect, had his agenda stalled in Congress when he was killed. Harry Truman left office deeply unpopular. Jimmy Carter lost re-election.

And Lyndon Johnson, despite grand domestic achievements, was driven from office. The chant “Hey, hey, L.B.J., how many kids did you kill today?” doesn’t exactly suggest progressive heroism.

This history of liberal disappointment was the subtext of a revealing early comment from Obama: “Reagan changed the trajectory of America in a way that Richard Nixon did not and in a way that Bill Clinton did not.” The history also led Obama to reject the advice of his first Treasury secretary that their legacy should be preventing another depression. “That’s not enough,” Obama replied.