Good point. I wonder how much of that is due to the long period of peace and prosperity that the U.S. has experienced; in the absence of any serious external threats, it becomes easier for partisans to squeeze their political opponents into that role.

9/11 was an anomaly. Following the attacks we saw a time of genuine good will between the parties. The further we get from that date, the more our politics is descending back into petty partisanship.