Mr. Blair, making a brief visit to Washington, also lent Mr. Bush his support on the question of whether the president had misled the American public in the weeks leading up to the war by including in the State of the Union address an allegation, attributed to British intelligence, that Iraq had tried to acquire uranium in Africa to restart its nuclear weapons program.

He said he stood by the intelligence, even though the White House now says it was not firm enough to have been included in a presidential speech.

Mr. Blair also had something he wanted from Mr. Bush. The prime minister said he would discuss with the president concern in Britain that British citizens captured in Afghanistan were being held by the United States at Guantánamo Bay as enemy combatants. The British government has made clear that it wants its citizens, including two scheduled to come to trial soon before a military tribunal, to be returned home to face trial there. Mr. Bush said he would discuss Mr. Blair's concerns.

Despite Mr. Blair's suggestion, in a speech to a joint meeting of Congress, that the war in Iraq would have been worth fighting even if no chemical, biological or nuclear weapons are found, Mr. Bush later insisted at a news conference with the British leader that they would be found. The president said that would "end all this speculation" about "whether or not the actions were based on valid information."

On a day when the White House came under increasing political pressure from Democrats to explain more fully how the passage about Iraq's attempts to buy uranium got into the State of the Union address, Mr. Blair's presence here helped the administration's effort to shift attention to the broader question of whether the world is better off with Mr. Hussein gone.

"I really don't believe that any responsible leader could ignore the evidence that we see and the threat that we face," Mr. Blair said today at a news conference at the White House after his speech on Capitol Hill. 

With Mr. Blair at his side during the news conference, Mr. Bush turned a question about whether he took personal responsibility for including the disputed information in the State of the Union address into an opportunity to applaud his own performance as commander in chief.

"I take responsibility for making the decision, the tough decision to put together a coalition to remove Saddam Hussein, because the intelligence — not only our intelligence but the intelligence of this great country," Mr. Bush said, referring to Mr. Blair's Britain, "made a clear and compelling case that Saddam Hussein was a threat to security and peace."

Mr. Blair, the fourth British prime minister to speak to a joint meeting of Congress, was invited in connection with being awarded the Congressional Gold Medal.

The British leader's presence in the Capitol created a political paradox: Mr. Blair, whose Labor Party would be considered to the left of the Democratic Party in the United States on most issues, received a rousing welcome from a Republican-controlled Congress. Republicans were not only expressing their appreciation for his stalwart support of the United States in confronting Iraq and the broader threat from terrorism, but were also welcoming him as a bulwark against attacks, mostly from Democrats, on Mr. Bush's credibility.

But he won over members of both parties with a self-deprecating reference to his own deepening political troubles at home, noting that he was not used to sustained applause. He brought down the house by noting that Senator Bill Frist of Tennessee, the Republican majority leader, had pointed out to him the fireplace in which the British had burned books from the Library of Congress in 1814.

"I know this is kind of late," Mr. Blair said with a comic's timing, "but: sorry."