Barack Obama's speech acquired a title nearly as soon as it was delivered. On both the campaign website and YouTube, where it has been seen more than two million times, it was identified as "A More Perfect Union."

The four words refer, of course, to the preamble to the Constitution, which was appropriate both as a gesture to Obama's hosts (Philadelphia's National Constitution Center, conveniently located in Pennsylvania) and as a reflection on our very imperfect society. It is a strange phrasing -- grammatically impossible, as some high school English teacher is doubtless pointing out somewhere, because nothing perfect can be either more or less perfect. But it sounds good, as the founders surely knew as well. The Constitution needed some gloss, both because its language is dull and because its politics stop notably short of perfection (the three-fifths clause, for example). Who wrote these famous four words? According to Carol Berkin, author of A Brilliant Solution: Inventing the American Constitution, it was none other than Gouverneur Morris, the brilliant one-legged New Yorker who was actually a delegate from Pennsylvania at the time the document was created. Years later, he wrote to a friend that the Constitution "was written by the Fingers which wrote this letter."

Obama was obviously right to choose these four words for his title, because they sound uplifting, as he wanted them to be, and historic, as he undoubtedly is. Many candidates and presidents have traveled to Philadelphia to speak on national issues, but they nearly always go to Independence Hall to summon the ghosts of the Declaration -- which, let's face it, is a braver and more idealistic statement of our founding principles than the Constitution's rules of what to do and not to do. Lincoln gave a haunting, short speech there in 1861, on the way to assume the presidency of a broken nation, and Woodrow Wilson went there in 1917 to explain why America was about to go to war.

Despite the generally favorable response of pundits to the speech, few seemed to note that the phrase was historic in several senses, conjuring Bill Clinton as much as it did the founders. Clinton loved the phrase, dropping it into the end of dozens of speeches -- instinctively understanding that it perfectly summed up a society that is perpetually in the act of becoming. By that, I don't only mean that we change our politics from week to week -- a fact that this campaign has made plain. Rather, one can argue that each generation has improved on the one preceding it. As recently as 2004, at the very convention that gave Obama his national platform, Clinton returned to the phrase in his hugely popular speech. CNN even titled it, "Time again to choose a more perfect Union." But four years is forever in politics, and apparently no one can remember that far back.

One of the strangest elements of the current debate between two very strong Democratic candidates is the degree to which the one who is considered the gifted orator has borrowed extensively from the speeches of his rival's husband. I don't mean plagiarism, which was last month's issue, but rather that the message of "hope" is not exactly new in the marketplace, and also that many of Bill Clinton's signature thoughts have been used in other Obama speeches, such as asking the black community to take more responsibility for its problems. That was the theme of what is still remembered as one of the great Clinton speeches -- his remarks at Mason Temple in Memphis on November 13, 1993. Obama has expressed the same thought on several occasions, including his impressive speech on civil rights in Selma a year ago. That's not wrong -- a good idea should be repeated -- but the job of a historian is to note convergences, and there are plenty to note in the current climate.

The worst part of Obama's idealistic speech was its least idealistic moment, when he lingered over South Carolina and the accusation that "some" had polarized the racial debate there. As Sean Wilentz argued in these pages, that claim is becoming more and more suspicious, and may even have been trumped up by Obama supporters. But once the candidate moved past that unsavory bit of political business, the speech lived up to expectations, and probably even exceeded them.