The passage of US healthcare reform is enormously consequential, and not just for the things the new law aims to affect. The manner of its passage, as much as its substance, makes November's mid-term elections pivotal. They may decide the political trajectory of the United States for the next several decades.

To see why, you must understand the improbability of what has happened. Barack Obama just tore up the US political rulebook.

A year ago there were two scenarios for healthcare reform. One was that the Democrats would carry a willing public with them and pass a comprehensive bill. Another was that opinion would cool, forcing the Democrats to settle for less. What happened was extraordinarily unlikely: the public turned against the Democrats' proposal and the party went ahead and did it anyway.

In Europe, rule by a political class that tells voters what is good for them is an idea so familiar that it is quite taken for granted. In the United States it is novel, and not instantly welcome.

Between now and November, Democrats must persuade the country that they acted in its best interests when they overrode the public's doubts. If they succeed and retain their majorities in Congress they will have a green light to advance their wider aims, which include tax reform, labour relations, energy and industrial policies. They will conclude that Clintonism, with its submission to centrist opinion, was an error: they will have learned that they can capture and move centrist opinion. But if voters punish their arrogance, their momentum will be stopped. US policy will be set on a very different course.