If you needed confirmation that American liberals and conservatives inhabit parallel universes, just consider the reaction to last week’s deal on the fiscal cliff. Conservatives saw a ruthless Barack Obama, ramming through his agenda. Liberals lamented a limp surrender by the president.

But these two very different lines of attack on Mr Obama have something important in common. Both liberals and conservatives remain addicted to the myth of the imperial president. They expect the occupant of the White House to be a towering figure – a Lincoln, a Roosevelt, a Johnson – who dominates politics and shapes history.

In reality Mr Obama is a prisoner of circumstances. He cannot pass laws or get a budget through without the consent of a House of Representatives that is controlled by his bitter ideological and political foes. No president – however brilliant, determined or wise – could create rational and effective policies from such circumstances.