The mainline Protestant churches have lost members, while more Americans identify themselves as secular or as evangelical Christians. Increasingly, we choose to live in neighborhoods and metro areas dominated by those who share our cultural and political views.
The bipartisan agreement on foreign policy that prevailed for two postwar decades ended as doves came to dominate the Democratic Party, while hawks became Republican. The abortion issue, which split both parties' constituencies in the 1970s, in time became a defining issue for both parties, as pro-lifers abandoned the Democrats and pro-choicers the Republicans.
All of which leaves little room for centrists, who in any case are a diverse lot -- libertarians who are conservative on economics and liberal on cultural issues, traditionalists who are liberal on economics and conservative on cultural issues. You can find a few members of Congress who fall in those camps, but not many.
The polarization of our politics is increased somewhat by partisan district lines. But overall, it's a reflection of our society and a result of the increasing intrusiveness and involvement of government in areas of life that used to be left alone. Changing longstanding laws on abortion and gay rights was bound to stir controversy and heated involvement on both sides. Issues of war and peace naturally arouse strong partisan views.
In the last 16 months, the Obama Democrats' proposals to vastly increase the size and scope of the federal government and to put federal spending on the way to doubling the national debt as a percentage of the economy have tended to sweep these cultural and foreign policy issues aside. They have increased the polarization of the parties, but have also produced some Democratic primary battles between supporters and opponents of the Obama program. The result could be a little less polarization -- but don't count on it.