[Democratic] policy unity has been helped by the fact that Obama has had a moderate degree of success in achieving these goals. If he had had an easy time, the party might be divided between those wanting more radical action and those not in a hurry; if he had failed utterly, the party might be divided (as it was for much of the past three decades) between a liberal faction and a Republican-lite faction. As it is, however, Obama has managed to achieve a lot of what Democrats have sought for generations, but only with great difficulty against scorched-earth opposition. This means that the conflict between "the Democratic wing of the Democratic Party" - exemplified these days by Elizabeth Warren - and the more pro-big-business wing is relatively muted: the liberal wing knows that Obama has gotten most of what could be gotten, and the actual policies haven't been the kind that would scare off the less liberal wing.
Krugman is responding to Matt Yglesias's argument that Hillary Clinton's broad support is the result of an unusually unified Democratic Party, not the cause of it. You should read that, too.