WASHINGTON — Steadfast allies President Bush and Prime Minister Blair yesterday strongly defended their decision to go to war in Iraq and to remain there, despite rising opposition in both America and Britain and their own plunging approval ratings.

“This is a fight we cannot afford to lose,” Mr. Blair said.

Standing side by side in the Rose Garden, the two leaders said they had no regrets about the decision, contending Iraq has become the main battleground in the war against global terrorism.

“It is an important part of protecting the United States,” Mr. Bush said. “No matter how calm it may seem here in America, an enemy lurks, and they would like to strike, they would like to do harm to the American people.”

Said the outgoing British prime minister: “The forces that we are fighting in Iraq — Al Qaeda on the one hand, Iranian-backed elements on the other — are the same forces we’re fighting everywhere.”

Mr. Bush said he’s optimistic a compromise will be reached with Congress on an Iraq spending bill. “I think we’ll get a deal. We’ll work through something we can all live with,” Mr. Bush said.

Congress and the White House have been at loggerheads over war spending since earlier this month, when Mr. Bush vetoed Iraq spending legislation after the Democratic-controlled Congress added provisions for troop withdrawals to begin October 1.

Focusing on Iraq at what seemed certain to be their final White House meeting, Messrs. Bush and Blair participated earlier yesterday in a secure video conference with U.S. and British officials in Baghdad for an update on security, economic, and political issues.

The president took the prime minister to the Situation Room in the basement of the White House for the hour-long briefing from General David Petraeus, the top U.S.commander in Iraq, U.S. Ambassador Ryan Crocker, and their British counterparts.

Mr. Blair, stepping down as prime minister on June 27, spent the night at the White House, a mark of the special relationship he has with Mr. Bush. Many believe that friendship tarnished the legacy of the once-popular British leader. Mr. Blair stayed in the Queen’s Bedroom, used by Winston Churchill during the former British leader’s frequent World War II-era visits to Washington.

Mr. Blair predicted that Britain would continue to stand side by side with America after he leaves office. “I believe that we will remain a staunch and steadfast ally in the fight against terrorism,” he said.

Mr. Blair, once enormously popular in his country, saw his popularity tumble largely over his alliance with Mr. Bush on Iraq.

The president praised Mr. Blair, calling him extremely effective as a leader and “dogged” when he gets on a subject. “I appreciate the fact that he can see beyond the horizon. And that’s the kind of leadership the world needs,” Mr. Bush said.

Asked by a British reporter if Mr. Blair was the right person for Mr. Bush to be dealing with now, given that he will leave office on June 27, Mr. Bush said absolutely. He admonished the reporter for “trying to do a tap dance on this political grave” and said, “You don’t understand how effective Blair is.”

Mr. Blair said he was proud to stand shoulder to shoulder beside Mr. Bush since the September 11, 2001, attacks: “I admire him as a president and I regard him as a friend.”

Mr. Bush voiced optimism that he could reach a deal with Congress on a stalled $124.2 billion spending bill to help pay for U.S. troops in Iraq and Afghanistan.