Saying a sacred institution is under threat, President Bush is summoning Congress to pass a constitutional amendment barring same-sex marriage.
He says judges and activists in three states have sought to overturn two centuries of US law, as well as the clearly expressed will of the people.
This, says Bush, has "created confusion on an issue that requires clarity."
So the president tells reporters he wants lawmakers to move quickly on an amendment that defines marriage as between a man and woman.
Bush called it "the most fundamental instutition of civilization."
However, Bush isn't endorsing specific wording. And he says any amendment should leave states free to allow same-sex civil unions.
Bush says the Constitution should never be amended "lightly." But he says "preservation of marriage rises to this level of national importance."