WASHINGTON — President Bush warned Americans on Tuesday night that there could still be a terrorist attack on the United States and then presented the nation with a choice between his continued leadership and a return to the "dangerous illusion" that the threat had ended.

In a nearly hourlong, prime-time State of the Union address, Bush showcased the extent to which he will use his administration's fight against terrorism in his re-election campaign. But he then moved on to a variety of domestic issues, making his strongest denunciation of gay marriage, an issue of great concern to his most conservative supporters.

"Our nation must defend the sanctity of marriage," Bush said.

Bush also offered a panoply of smaller domestic proposals, such as a plan to promote abstinence among teenagers, increased funding for drug-testing in schools and a call for athletes to stop using steroids.

In a reflection of the restrictions imposed on him by the growing budget deficit, many of the proposals he set out carried relatively little or no cost. He never discussed one of his most potentially costly new proposals, his declaration last week that Americans would return to the moon by 2020 and use that as a launching pad to Mars.

Bush cast himself as the steady commander-in-chief of what he portrayed as a nation at war, seeming to suggest that changing the leader mid-battle was a risky course.

"Twenty-eight months have passed since September the 11th, 2001, over two years without an attack on American soil, and it is tempting to believe that the danger is behind us," Bush said, in the somber tones he reserves for his most important speeches. "That hope is understandable, comforting and false."

"We have not come all this way, through tragedy and trial and war, only to falter and leave our work unfinished. Americans are rising to the tasks of history, and they expect the same from us."

Bush put forth no major new foreign policy or domestic initiative in an address that was as much about political drama as substance, and which served as the president's grandest stage until his party's nominating convention in New York in September.

Speaking to an estimated audience of 60 million people, in an address that the White House deliberately scheduled a day after the caucuses in Iowa and a week before the primary in New Hampshire, Bush defended himself on a wide range of foreign and domestic matters, from tax cuts to Iraq to whether the war on terrorism is, as he put it, "a war at all." Bush most directly took on his critics over the war in Iraq and the presence of banned weapons there.

"Some in this chamber, and in our country, did not support the liberation of Iraq," Bush said. "Objections to war often come from principled motives. But let us be candid about the consequences of leaving Saddam Hussein in power."