BETHESDA - With America still held rapt by a fallen space shuttle and the deaths of seven astronauts, President Bush shifted attention yesterday, ever so delicately, to matters that he says carry grave urgency, notably homeland security and the threat from Iraq.

Even as he comforts a shaken country, aides say, Bush is determined not to let the national tragedy of the space shuttle Columbia imprison him or his agenda.

Visiting the National Institutes of Health here for a previously planned event, the president offered more words of grief and prayer about Columbia but only for about a minute.

Then he turned to the Sept. 11, 2001, terrorist attacks, conjuring memories of that catastrophe, which is more directly related to Bush's top priorities than is Saturday's space shuttle disaster.

'Dangers of new era'

The terrorist attacks 17 months ago, Bush said, "awakened America to the dangers of a new era" and showed why, for example, a nation such as Iraq must be disarmed, before Saddam Hussein can endanger America or allow weapons of mass destruction to fall into the hands of terrorists.

"The kind of men who would seize planes filled with innocent people and crash them into buildings," Bush said, "would not hesitate to use biological or chemical or nuclear weapons. They wouldn't hesitate at all."

Any president must perform a careful balancing act when confronted by a national calamity - soothing the collective pain but also seeming strong, clear of vision and able to concentrate on the nation's pressing problems.

Especially for Bush, the space shuttle accident came at a time already freighted with significance, as he faces two huge challenges - the possibility of war against Iraq and a stubbornly sluggish economy - that could define him as a leader and the ultimate success of his presidency.

Still determined

Aides stress that, despite the shuttle accident, Bush's determination to confront Hussein has not wavered.

Secretary of State Colin L. Powell, as scheduled, will travel to the United Nations tomorrow to present what administration officials say is intelligence that Iraq possesses weapons of mass destruction and that Hussein has links to terrorists.

The Columbia disaster also came two days before Bush was to issue his new federal budget plan, a document that he sent to Congress yesterday and that largely enumerates his priorities as president.

One senior White House official called it Bush's "responsibility to the country" to ensure that, as he guides the nation through a period of mourning, he does not become distracted from issues that he has described to Americans as vitally important.

"When you are the president of the United States, you always face a number of challenges - some of which you choose, and some of which you accept," the official said.

Asked why Bush, 48 hours after the space shuttle broke apart, decided to hold an unrelated event in Maryland, the official said: "Life continues on."

The president tried to signal yesterday that he could simultaneously manage a crisis and press on with his agenda.

Briefings on disaster

In the morning, he was briefed by Sean O'Keefe, NASA's administrator, about the status of the early inquiries into why Columbia ripped apart in the sky. Bush also received updates all day from his chief of staff, Andrew H. Card Jr.