Barack Obama has become the first African-American with a serious chance to be elected President of the United States. And he had already made history before that, by becoming in 2004 only the third black elected to the U.S. Senate since Reconstruction. (The other two were Edward Brooke, elected in Massachusetts in 1966 and 1972, and Carol Moseley Braun, elected in Illinois in 1992. In addition, two blacks have been elected governors of states, Douglas Wilder in Virginia in 1989 and Deval Patrick in Massachusetts in 2006.) But Obama’s rise points to a puzzle that begs explanation.

Blacks currently hold 38 districts in the U.S. House of Representatives, 9 percent of the total, and about 600 seats in state legislatures, about 8 percent of the total. That is below, but not all that far below, the 12 percent that is the black percentage of the population. But in the 722 elections for U.S. senator and the 579 elections for governor held since the passage of the Voting Rights Act of 1965, blacks have been elected in only four of those contests, which works out to just three-tenths of 1 percent. What explains this discrepancy between the percentage of blacks elected in legislative districts and the percentage elected to the highest statewide offices?

One might ascribe the gap to white racism. No state has a black majority, and in the state with the highest black percentage, Mississippi, the 63 percent of whites vote overwhelmingly one way and the 37 percent of blacks vote overwhelmingly the other way in high-visibility elections. That has been true not only in general elections, but also in the 2008 Democratic presidential primary between Hillary Clinton and Senator Obama.

Another explanation might be that blacks do not run for major statewide offices very often. The list of serious black competitors for governor or senator since 1965 is not much longer than the list of those elected. The most recent example is Congressman Harold Ford, who won 48 percent of the vote in Tennessee in 2006. Other examples include Senators Brooke and Moseley Braun, who won large percentages of the vote but were defeated for re-election in 1978 and 1998, respectively. As long ago as 1972, Congressman Andrew Young was elected in a majority-white district in Georgia. The impressive showing of Senator Obama in the 2008 presidential primaries and the poll results that show him running roughly even with John McCain—and about as strongly as Hillary Clinton—are evidence that white Americans are ready to...