Politics - John Kampfner asks if Blair will fall like Thatcher

Iraq will be the defining issue of Blair's premiership. Has he become like Margaret Thatcher in her

Triangulation is one of Tony Blair's favoured tactics. Identify the two opposing viewpoints, incorporate parts of them and determine policy in your image somewhere in between. On Iraq, however, he has a problem. Labour's core support, in the constituencies and the unions, has found unlikely common ground with the politically non-aligned of Middle Britain. Not much room for triangulation there.

So Blair's speech to the Trades Union Congress was a difficult one to craft. The sideswipes at what Downing Street calls the new generation of "syndicalist" leaders over their "self-indulgent rhetoric" were easy to make. But for the first time - on Iraq - Blair knew he could not turn the unions' resentment of him to his favour. He's read the polls. He knows those concerns are shared more broadly.

Still, the Blairites were appalled by the Iraq debate on the first day of the TUC conference. The content and tone of the speeches were uncompromisingly hostile. "We thought we'd seen the last of big fat men shouting and thumping at the microphone," said one government official. "It's as if all our work with good people like John Monks has come to nothing."

They were preparing for worse the next day. Blair spent nearly 24 hours huddled in his suite of rooms, meeting the general secretaries of the major unions. Word spread that various groups, notably Unison, were preparing protests against Blair. The PM's team heard that one group of delegates had been practising a routine in which they would stand up in the middle of the speech and then turn their backs on him. Others were said to be preparing heckles and choreographed walkouts.

In the event, and in keeping with TUC tradition, Blair was heard out with only the meekest of mutterings against him. But the respectable behaviour masks a far deeper tension. "We have a psychology problem," said one Blair aide. "Whatever we do for them in industrial relations or on the economy, they ignore; whatever we don't do, they remember."

On foreign policy, that psychology problem extends further. There is something visceral in the fear and loathing of George W Bush and it is expressed far beyond the confines of TUC or Labour gatherings.

One minister, part of an unusually large ministerial delegation despatched to Blackpool, described the American president to me as "evil". An MP said Bush had "looped the loop", and that in his enthusiasm to stand side by side with him, Blair "risks looking a bit unhinged". If only, several of them said, this was still the Clinton era. Military action would then be easier to sell. Blair alluded to that when he addressed the TUC gala dinner after his speech, raising a laugh by sending "fraternal" greetings from Bush.

The chronology of events in the coming weeks is becoming clearer. First, Bush will address the United Nations, setting out the terms and deadline for Iraqi compliance. Then Downing Street will release its "dossier" on Iraq's chemical, biological and nuclear capabilities. This will be followed by a recall of parliament (as Blair conceded to the TUC), though almost certainly the government will not allow MPs a substantive vote.

The hope is that by the time Blair returns to Blackpool for the Labour Party conference later this month, his critics will appreciate the scale of the diplomatic efforts that would precede a military attack.

Party officials accept that Iraq will dominate the Labour conference; but there is an argument between those who advocate a full debate on the day before Blair's speech, to get it all out into the open, and those who want to hold it back until the day after.

Blair has only just begun to make the case for conflict with Iraq, combining fears of proliferation of weapons of mass destruction with his newfound doctrine of liberal imperialism to rid the world of illiberal leaders. The Blairites' frustration is palpable. But they proved the doubters wrong on Kosovo and Afghanistan, and they believe they will do so again now.

If it were only the "big fat men" expressing their misgivings about Blair's Iraqi adventure and his unlikely alliance with Bush, he wouldn't have to worry so much. Just as Clause Four became the defining issue for Blair in opposition, so Iraq, it seems, will be the defining issue of his premiership. For the first time, some in the senior ranks of the party and the union movement, even some in government, are beginning to question Blair's judgement.

Comparisons are being made with the final years of Margaret Thatcher. By the end, she was picking the wrong battles to fight. As a senior member of the government entourage put it to me: "In a year's time, either we will have been right, and we'll let them know it, or we'll have been wrong and God help us."