The aftermath of the attempted bombing has been marked by an increasingly fierce partisan exchange over culpability heading into a midterm election year. With Republicans on the attack against the administration as not taking terrorism seriously enough, Democrats returned fire by accusing the opposition of standing in the way of needed personnel and money while exploiting public fears.

The debate has escalated since Mr. Obama’s secretary of homeland security, Janet Napolitano, said Sunday that “the system worked” after officials said the suspect tried to ignite explosive chemicals aboard a Northwest Airlines flight approaching Detroit. Ms. Napolitano made clear the next day that she had meant the system worked in its response to the attempted bombing, not before it happened.

Mr. Obama appeared to be trying to contain the damage on Tuesday, offering “systemic failure” as a substitute diagnosis for “system worked.” He framed Ms. Napolitano’s statement by saying she was right that “once the suspect attempted to take down Flight 253, after his attempt, it’s clear that passengers and crew, our homeland security systems and our aviation security took all appropriate actions.”

The president praised the professionalism of the nation’s intelligence, counterterrorism, homeland security and law enforcement officials. But he spared little in his sharp judgment about how a known extremist could be allowed to board a flight bound for the United States after his own father had warned that he had become radical.

“There was a mix of human and systemic failures that contributed to this potential catastrophic breach of security,” Mr. Obama told reporters at the Marine Corps base at Kaneohe Bay outside Honolulu, near his vacation home in Kailua. “We need to learn from this episode and act quickly to fix the flaws in our system because our security is at stake and lives are at stake.”

Mr. Obama suggested that he would overhaul the watch-list system. “We’ve achieved much since 9/11 in terms of collecting information that relates to terrorists and potential terrorist attacks,” he said. “But it’s becoming clear that the system that has been in place for years now is not sufficiently up to date to take full advantage of the information we collect and the knowledge we have.”

Mr. Abdulmutallab, who has been linked to the Yemeni branch of Al Qaeda, came to the attention of the American authorities when his father went to the American Embassy in Nigeria last month to report that his son had expressed radical views before disappearing. The father, a respected retired banker, did not say his son planned to attack Americans but sought help locating him and bringing him home, United States officials said.