Prime Minister Tony Blair is facing his toughest political battle since taking office over growing opposition to his support for President Bush and U.S.-led military action against Iraq, according to analysts and opinion polls.

Blair, America's staunchest international supporter, has been hit by a series of setbacks in recent days, starting with an equivocal report by the chief U.N. weapons inspector, Hans Blix, to the Security Council last Friday. That was followed by Saturday's mass demonstration opposing a war, a protest now said to have attracted more than 1 million people, the largest political rally in British history. This morning, a new opinion poll showed Blair at his lowest approval rating in 21/2 years.

Other U.S. allies in Europe also face political difficulty. In Italy, where an estimated 1 million marchers protested against military action and Prime Minister Silvio Berlusconi's support for the Bush administration, the country's main labor union, the CGIL, today threatened to launch a general strike if war broke out. In Spain, where 2 million to 3 million people took to the streets Saturday, Prime Minister Jose Maria Aznar's party has fallen behind the rival Socialists in opinion polls for the first time since he came to power three years ago. Aznar is to meet with Bush this weekend at the president's ranch in Crawford, Tex.

Blair and his aides are disturbed that despite a month-long blitz of public appearances in which he has sought to make the case against Iraq, opposition to military action is increasing. Blair's opponents within his ruling Labor Party have become increasingly emboldened and for the first time are talking openly of seeking to replace Blair if he takes Britain into war without a second Security Council resolution authorizing force.

"This is crunch time for Tony Blair," said Alan Simpson, a leader of Labor's antiwar faction in the House of Commons. "He can lead the war party or the Labor Party, but he can't lead both. It's quite clear if he goes off to war, he will have left the party behind him."

At the same time, Blair is pressing the Bush administration to return to the Security Council for another resolution before launching an attack. "I still believe we should have a second resolution," Blair said at a news conference today. But he declined to comment on the timing.

A U.S. official said the operative U.N. resolution "has everything we think we need, and we probably don't need a new one. But it's increasingly obvious to folks in all capitals that a new resolution would be a very good thing. And it has to be said that Blair politically really needs it."

At the news conference, Blair acknowledged the depth of political opposition and pleaded with Britons to give him a fair hearing. "It's one of these issues where you've got a duty to try and say to people what you believe," he said. "Look, I don't pretend to have a monopoly of wisdom in these issues, or that I always know what's right and everybody else is wrong. I totally understand why people march and oppose what we're doing. I just ask people to listen to the other side of the argument." He also said, "There is no rush to war."

Blair has committed 40,000 troops to a prospective campaign against the Iraqi government of President Saddam Hussein. Analysts say Blair's campaign to win support has faltered for a variety of reasons, beginning with a high level of public concern and alarm over the Bush administration and the use of U.S. power. A poll last month reported that 74 percent of Britons surveyed had "not much" confidence or "none at all" in Bush. Blair generally has stopped mentioning Bush's name in his speeches, although he spoke about Bush today in defense of Britain's position.

"Don't look at the parody of what George Bush has done, look at the reality," Blair told reporters, noting that Bush had gone to the United Nations in September to seek its support against Iraq. "Some of the rhetoric that I hear used about America is actually more savage than I hear used about the Iraqi regime. Come on, let's get a sense of perspective here."

Blair has failed to convince the British public that the Iraqi government is a direct and imminent threat to Britain, and has not connected the campaign against Iraq with the war on terrorism, analysts said. He is also suffering from a high level of public mistrust. The government's release two weeks ago of what was billed as an intelligence dossier on Iraq turned out to be largely copied from a graduate student's report, and it damaged Blair's credibility. Some politicians have even suggested that the security alert that sent troops and tanks to Heathrow airport last week had been issued to rally support behind the government -- a charge officials indignantly denied.

Still, said Adam Roberts, professor of international relations at Oxford University, the government's stance on Iraq and fighting terrorism requires a high degree of public confidence. "They're making decisions that are intelligence-related and the public must take a lot of it on trust," he said, "and that just isn't there."

The poll published today, conducted by ICM, showed 52 percent opposed to military action, with 29 percent in favor and the rest undecided. Meanwhile, Blair's approval rating has dropped 9 points in the past two months -- to 35 percent, the lowest level since September 2000, when Britain suffered a gasoline shortage.

"It's his hardest moment," said Michael White, political editor of the Guardian newspaper, which published the poll. "He's running against public opinion. A lot of people who don't like him anyway are out to get him, and a lot of people who do like him simply aren't persuaded."

Blair insists he can still talk people into supporting him -- even many of those who marched against him Saturday. "A lot of people out there I would put in the unconvinced category rather than the never-be-convinced category," he said.

Bob Worcester of the MORI public research firm agreed, saying Blair would readily overcome his problems if a new U.N. resolution were passed and military action against Iraq succeeded.

But Roberts added a cautionary note. "If the crisis is resolved within the government's parameters of success, then he'll be okay," he said. "But he's gone out very far on a limb, and if there's a failure -- and it can be failure as defined in many different ways -- then he's got a serious problem."

Tony Blair defends position on Iraq during monthly news conference.