This week’s last-ditch health care push may or may not prove the defining battle of Barack Obama’s presidency. It may or may not prove a defining moment in the history of the American welfare state. But here’s a good bet: The Democratic Party will never be the same.

For close to a decade now, Democrats have been arguing with each other about what kind of country this is, and what kind of party they should be. On one side stands a group of politicians, consultants and wonks who believe that America is, at its core, a pretty conservative place. These Democrats form something of a political generation. In their youth, they saw their party move left during Vietnam and get booted from power in 1968. Then they saw George McGovern, the most left-wing major party...