From the 1930s to the 1960s, hundreds of thousands—maybe millions—of American homes contained two pictures: one of Jesus Christ, the other of Franklin D. Roosevelt. By the late 1960s, FDR's image was giving way to pictures of John F. Kennedy and, especially in African-American homes, of Martin Luther King Jr. Their images still often enjoy pride of place with Jesus.

Now Barack Obama is seeking a spot on that wall some day. In late August, he will accept the Democratic nomination in a 75,000-seat outdoor football stadium in Denver—just as JFK did at the Los Angeles Coliseum in 1960, where he unveiled his idea of a "New Frontier" before a huge crowd. In all the years since, no other nominee in either party has moved out of a much-smaller indoor arena on the fourth night of his convention, as Kennedy did. But the symbolism doesn't end there. Aug. 28, the date of Obama's acceptance speech, marks the 45th anniversary of King's "I Have a Dream" speech in front of a throng gathered at the Lincoln Memorial for the 1963 March on Washington. Neither the setting nor the timing is a coincidence.

These are powerful historical currents Obama is swimming in, and potentially treacherous ones. He evokes Kennedy's cool and political freshness, and King's promise of deliverance from America's original sin. But the more Obama encourages the comparisons, the higher he drives expectations. Even if the night goes well (and it doesn't rain), Obama's speech will be measured not only against his own best performances, but against those of a time when we imagine inspirational giants roamed the earth. The Democratic nominee will confront two of the most powerful political ghosts in our history.

This is an impossible standard to meet. How can Obama compete with idealized figures whose shortcomings have been shrouded in the mists of memory?

Of course, these men were not so popular in their day. In 1960, for instance, Eleanor Roosevelt remarked that Kennedy should show "a little less profile and a little more courage" (some liberals are saying the same about Obama now). She didn't fully support him until after Labor Day. Even after being elected, JFK was slow to act on civil rights. While he complimented King on his "Dream" speech, he shied away from attending the march.

King, too, has benefited from history's gloss. He preached nonviolence, but was not nearly radical enough for many black nationalists. At the same time, today's conservatives have forgotten that many of them—or their parents—thought King was a communist.