Jonathan Rauch is a guest scholar at the Brookings Institution and author of Gay Marriage: Why It Is Good for Gays, Good for Straights, and Good for America.

...Obama’s gay-marriage conversion smacks of conviction, not convenience. Waiting until after the election would have been politically safer, but if Obama loses in November, as he knows he might, a historic opportunity to speak out for justice could have slipped away. President Clinton has said he regrets having signed the Defense of Marriage Act. Obama seems to have decided not to repeat the error.

“We shall overcome,” LBJ said in March of 1965, shortly after his reelection. When he said those words, he knew he was writing himself into the history books. But he also knew he would probably be writing off the South. There’s no doubt that Obama is a more cautious politician than Johnson: If he thought helping gays would have cost him the election, he wouldn’t have done it—and gays wouldn’t have wanted him to. And the political risk he is taking is not of the same magnitude as LBJ’s. The country has come far enough on marriage equality to make a stand on principle affordable. African-American equality was unique in its moral importance and political voltage, so Johnson’s gesture continues to stand as unique, and, we must hope, always will.

Still, Obama has claimed for himself a place in gay history not unlike LBJ’s place in black history. He is the first U.S. president to put the federal government unequivocally on the side of full equality for gay Americans, and he will almost surely be the last Democratic president to have opposed full equality. For his party, for its liberal base, and possibly for the country, there is no going back. He has crossed the bridge from Selma.