A revealing anecdote during last fall's remembrances of the Kennedy assassination described how Lyndon Johnson, the slain president's successor, bullied Sen. Richard Russell of Georgia into serving on the Warren Commission to investigate the shooting.
Russell was a powerhouse conservative and segregationist. Johnson figured he'd balance the liberal commission chair, Supreme Court Chief Justice Earl Warren, maestro of a famously activist high court. Russell loathed Warren. Johnson didn't care.
But here's the historical oddity: Russell, the archconservative, was a Democrat. Warren, the liberal lion, was a Republican. Each belonged to a species that was common half a century ago but is now as extinct as the pterodactyl.
Consider three numbers from last week's Star Tribune Minnesota Poll: 0, 1 and 3.
The poll found that only 3 percent of Minnesota Democrats disapprove of the job Mark Dayton is doing as governor. Just 1 percent of Democrats disapprove of Sen. Al Franken's performance in office.
And Republicans? The poll reported 0 percent of them approving of President Obama's handling of his job.
These are staggering levels of groupthink. And without making too much of any single poll result, a wealth of evidence confirms the basic message here. Our political parties have become pathologically pure, harboring, at least on certain questions, no more intellectual diversity than a beehive.
It's almost unanimously agreed, of course, that America today is abnormally polarized, that we've seldom been so sharply divided. But is it possible that it's only our political parties that are more estranged than they used to be?