Democrats, for their part, have traveled in virtually the opposite direction. From the New Deal through World War II to the Great Society, the Democrats were the party of steady forward movement, of Big Ideas and Big Shoulders, the party that licked the Depression, won World War II, took the lead in creating the postwar international security establishment that would fight the Cold War, and aspired to fix the most daunting economic and social problems here at home. Liberal Democrats (in the North, not the South) were at the forefront of the civil-rights struggle (in crucial partnership with Republicans who remembered their party’s founding), which reached a legislative high-water mark in the mid-1960s with passage of the Civil Rights Act and the Voting Rights Act.

That very achievement split Franklin D. Roosevelt’s ruling coalition and, very quickly, delivered the Solid South to the Republicans for the first time since Reconstruction. Associated as they were with the programs of the New Deal and the Great Society, and with an expansion of rights for various groups through legislative and judicial action, the Democrats came across more and more as the crouched consolidators and defenders of past gains. When you realize that the ground you fought so hard to win is at risk, and that your first job is to hold it, you stop being an aggressor (and you start arguing with your allies, because not everything can be saved). In their new book, It’s Even Worse than It Looks, two of Washington’s leading congressional scholars, Thomas E. Mann and Norman J. Ornstein, put it this way: “The Democrats … have become the more status-quo oriented, centrist protectors of government, willing to revamp programs and trim retirement and health benefits in order to maintain the government’s central commitments.” Nearly 80 years after the New Deal ushered in a new understanding of government’s basic obligations, the current breed of Republican seems determined to re-litigate the consensus understanding of the social compact that has prevailed for generations, forcing Democrats to fight simply to stay in the same place.

The Big Flip is a central political fact of our era. Another is that the connecting tissue between the parties has disappeared. For much of the 20th century, ideological affinities between southern Democrats and conservative Republicans, on the one hand, and between urban and northern Democrats and moderate and liberal Republicans, on the other, not only allowed but effectively required cross-party cooperation to get anything important done.

Today there are no such cross-party incentives for cooperation.