A great op-ed piece today: a history lesson and a lesson on history-making.

As I watch Barack Obama’s speech to the Democratic convention tonight, I will be remembering another speech: the one that made Martin Luther King cry. And I will be thinking: Mr. Obama’s speech — and in a way his whole candidacy — might not have been possible had that other speech not been given.

That speech was President Lyndon Johnson’s address to Congress in 1965 announcing that he was about to introduce a voting rights act, and in some respects Mr. Obama’s candidacy is the climax — at least thus far — of a movement based not only on the sacrifices and heroism of the Rev. Dr. King and generations of black fighters for civil rights but also on the political genius of Lyndon Baines Johnson, who as it happens was born 100 years ago yesterday.

When, on the night of March 15, 1965, the long motorcade drove away from the White House, heading for Capitol Hill, where President Johnson would give his speech to a joint session of Congress, pickets were standing outside the gates, as they had been for weeks, and as the presidential limousine passed, they were singing the same song that was being sung that week in Selma, Ala.: “We Shall Overcome.” They were singing it in defiance of Johnson, because they didn’t trust him.

They had reasons not to trust him.