The question is being debated at the highest level in the Labour and Conservative parties: has Tony Blair reached the "tipping point" at which he irrevocably loses the trust of the people?
John Major "tipped" on Black Wednesday in 1992 and there was nothing he could do to regain the voters' confidence, although he clung on to power for four-and-a-half years. Now the best brains around Mr Blair are worried that the failure to find weapons of mass destruction in Iraq will prove the final straw for people already tiring of the Prime Minister.
"For some people, WMD will be the tipping point," one senior Labour figure told me this week. Labour's private polls suggest the issue is losing Mr Blair support among the professionals who flocked to him in 1997.
Tim Bell, the public relations man back in favour at Conservative Central Office, told the Shadow Cabinet before the Iraq war that Mr Blair was approaching his "tipping point". The Tories' latest focus groups show trust in the Prime Minister has now "gone though the floor" because people fear he took the country to war on a false prospectus.
The Tory research does not suggest the voters are ready to evict Mr Blair from Downing Street and install Iain Duncan Smith. Crucially, it does mean voters are ready to start listening to what the Tories are saying, after writing them out of the script for so long. The media, too, are taking the Tories more seriously: the "patients' passports" policy, which secured much coverage this week, was originally announced at last October's party conference.
Where does "the tipping point" come from? It is the title of a fascinating bestseller in 2000 by Malcolm Gladwell, a journalist in America. He says the "tipping point" is the magic moment when ideas, trends or social behaviour cross a certain threshold, tip, then spread like an epidemic. He shows how little things can make a big impact: removing graffiti on the New York subway led to a dramatic cut in crime. Sales of Hush Puppies were down to 30,000 pairs a year in the US until they became trendy in Manhattan's clubs and bars; 430,000 were sold the next year.
The worry for Mr Blair is that the theory relies heavily on word of mouth. Increasingly, the word on the street is that he cannot be trusted. This week's front cover of The Economist magazine shows a picture of Mr Blair with one word: "Bliar?". The Tory poster cannot be far behind, and Mr Duncan Smith's new soundbite is: "The trouble is, people don't believe him any more."
After his six years as Prime Minister, the voters are less likely to give Mr Blair the benefit of the doubt, as they did when he assured them he was "a pretty straight kind of guy" during the Bernie Ecclestone affair of 1997.
Iraq's WMD, or the absence of them, will not decide the next general election. But whether people trust Mr Blair could be a big factor in whether they give him a third term, and he will probably need one to achieve his ambition of taking Britain into the single currency. Although Gordon Brown will not rule out a referendum in this Parliament when he makes his long-awaited statement on the euro on Monday, he will not suggest one is likely before the next general election.
The trust factor could be even more important if Labour fail to deliver improvements in public services before the election. Many Labour MPs have tolerated him because he is a winner; if they think the public has turned decisively against him, then he could suffer a haemorrhage of support inside his party.
Yet, some WMD or evidence of them may turn up. The fear among some Blairites is that it will happen after the damage has been done, and Mr Blair will not be trusted on other issues. Perhaps the Prime Minister should read Gladwell's book before it is too late.
¿ John Prescott's V-sign to the cameras in Downing Street on Thursday has prompted speculation that he will be dropped or "kicked upstairs" to the House of Lords when Tony Blair reshuffles his Cabinet shortly. Forget it. Despite firing off his occasional loose cannonballs, the Deputy Prime Minister is valued by Mr Blair more than ever. He is a vital unifying force in a cabinet in which the rival egos provoke more divisions than policy differences. "He made a very impressive and important contribution to Thursday's discussion on the euro," one Cabinet minister told me.
Lord Irvine will probably retire as Lord Chancellor, though Mr Blair will not push him. Lord Falconer of Thoroton, Mr Blair's former flatmate, is the front-runner to become Lord Chancellor.
Other rumours are probably wide of the mark. Charles Clarke has been tipped for a move because of the school budget crisis, but I cannot believe that will happen only seven months after he became Secretary of State for Education. I suspect Alan Milburn, widely assumed to be moving from Health, will stay on to see through his reforms.
Geoff Hoon, the Secretary of State for Defence, is due for a move and so he may swap jobs with Patricia Hewitt, the Trade and Industry Secretary. The Welsh Office and Scottish Office, not needed after devolution, may be merged into a department for the regions. Peter Hain, the Secretary of State for Wales, can expect promotion.
Changes at Cabinet level may be limited, with a bigger shake-up amongst junior and middle-ranking ministers for new talent to replace long-servers and give the Government a fresh look. Will the voters notice much difference? Somehow, I doubt it.