And we have. After Bilbo, and despite a set of Southern Democratic senators who were more civil than he was but still tenaciously segregationist, Congress passed civil-rights bills in 1957 and 1964, and the landmark Voting Rights Act in 1965—thanks in large part to the efforts of Republican heroes like Bill McCulloch and Everett Dirksen. We have seen a sharp decline in racist attitudes, a widespread acceptance of interracial marriage, and many other salutary changes. But we are seeing vividly now that race remains a defining gulf in our society, despite remarkable progress over the past five decades.

Christopher Ingraham in The Washington Post recently showed a series of poll results reflecting major change. For example, in 1972, about two-thirds of whites said homeowners should be able to discriminate against blacks when selling their homes, but that was down to 28 percent in 2008. In 1988, two-thirds of whites said they would not be happy if a family member married a black person; that was down to 25 percent in 2008. Great progress, but the fact that over a quarter of whites still recently held racially prejudicial views is unsettling.

Americans of all stripes were justifiably proud when the country elected its first black president in 2008, and again when he was reelected in 2012. The fact is that no other comparable democracy, in Europe or elsewhere, was then or would now be prepared to elect a leader from a minority group. But even as I watched the celebrations on election night in November 2008, I felt an undercurrent of unease. Heartening as it was, this was not a sign that we had broken the back of racism or of racially driven divisions in the country. The election of an African-American president could be seen by racists in America as a sign that they could be more blunt in expressing their views. After all, who could now say America is racist? And the same mindset could lead others to enable statements or actions that would otherwise be seen as over the line. And, of course, the inevitable harsh criticism of a president by partisans on the other side, something that comes with the territory, could easily take on a racial dimension for Barack Obama.

It didn't take long for the latter phenomenon to emerge, with the birther movement, which on its face was ludicrous. To believe that Obama was not born in the United States meant that you had to believe that there had been a vast conspiracy 47 years earlier that included two Hawaiian newspapers that reported contemporaneously on his birth. A conspiracy, apparently, to enable a Kenyan-born Muslim plant smuggled in via Indonesia to be cultivated for decades as a Manchurian candidate to become president and destroy America as we know it. To be sure, delegitimizing a president has become a reality in the age of the permanent campaign—Bill Clinton, remember, was accused of being an accessory to murder—but to suggest that a president is a foreigner born in Africa takes it to a different level.