When will we give up on the idea of a leader who will magically bring consensus and unity to our politics?

At election time, candidates seduce us with promises to bring America together, but inevitably fall short and end up leaving office with the country more polarized than when they arrived. After blaming them for their failure to unite us, we turn to the next crop of presidential aspirants and the cycle of hope and disappointment begins all over again.

The latest example of this pattern, of course, is Barack Obama. His 2008 campaign was premised on his ability to forge new coalitions in Washington, which Hillary Clinton mocked at the time as “Let’s get everybody together, let’s get unified, the sky will open, the light will come down, celestial choirs will be singing.”

In office, though, he has never succeeded in overcoming Republican resistance to his agenda, forcing him to pass his most significant legislative accomplishments with few or no G.O.P. votes. After his party’s decisive defeat in last month’s midterm elections, the administration has now largely given up on bipartisan deal-making and is pursuing its policy goals on issues like immigration and the environment through executive action.