Tony Blair is temperamentally more of a president than a prime minister. He would like to be above the parliamentary battle, to concentrate on big domestic themes and to be a major figure on the world stage. That is the part he is playing with President Bush this week: the two great battlers against terrorism, just as Margaret Thatcher and Ronald Reagan were fellow crusaders against communism a generation ago.

Yet Mr Blair yesterday could not escape the prime ministerial reality, as opposed to the presidential aspiration. Yesterday, he alternated both roles, at Buckingham Palace and Westminster. But he was never allowed to forget that his political authority is as much tied up with the parliamentary votes on foundation hospitals and jury trials as with the Bush visit.

Indeed, this year, unlike his first term, Blair has had to be more of a prime minister than a president. He has not been able to bypass Parliament. He has faced record rebellions on Iraq and foundation hospitals -avoiding a defeat last night only because Labour MPs did not want to hand Michael Howard a morale-boosting early victory. The vote on tuition fees next month could be even closer.

Moreover, not only does Mr Blair share power with Gordon Brown over most of the domestic agenda, but he has to operate through Whitehall departments to implement policy. And while Mr Blair prefers to work bilaterally with key ministers, he has recently involved his Cabinet more, both on Iraq and more recently on identity cards and future strategy.

Blair casts himself as EU president candidate

Speaking on French TV, Tony Blair sang the praises of the EU and sketched his agenda for its future, all in fluent French

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There are striking parallels between Mr Blair's new political constraints and the dramatic events of 13 years ago. Mrs Thatcher had been attending a summit in Paris to mark the end of the Cold War and to consult with the elder President Bush about how to force Iraq out of Kuwait. But after failing to secure a sufficient majority in the first ballot of Tory MPs she had to hurry home to come to terms with the destruction of her domestic political base, and then to step down. American officials could not understand how such an apparently dominant leader could be forced out in this way, just as they were perplexed by the amount of time Mr Blair had to spend in the Commons yesterday.

Yet Mr Blair is even more presidential in his aspirations and style than Baroness Thatcher was. One senior civil servant who worked closely with both reckons that Blair would have liked to be a president -especially having seen, and admired, Bill Clinton's political skills. The television series The West Wing, starring Martin Sheen as the charismatic President, has become a must for eager Blairites.

The Downing Street operation has been reorganised and expanded to resemble the Executive Office of the President, albeit still on a much smaller scale. There is a chief of staff, a policy directorate, a strategy unit, directors of communications, government relations and political operations and what amounts to a national security adviser. We have a prime minister's office in all but name.

Moreover, Mr Blair has adopted a deliberately presidential style, trying to stay above the fray: no longer first among equals, but, rather, first above equals. He does his duty in Parliament, but he has developed the extraparliamentary side, via, for example, his monthly press conferences.

The Blair approach depends on "the power to persuade". This phrase was coined by the late Richard Neustadt, the engaging and original American student of the presidency and husband of Shirley Williams, who died three weeks ago. Neustadt's book Presidential Power, first published in 1960 and regularly updated, emphasised the limits of a president's formal powers, which are no guarantees of actual power or influence. These are more provisional and personal and depends on the informal exercise of power. But it is not permanent. In the British context, political scientists such as Michael Foley and Richard Heffernan have highlighted how Blair, like Thatcher, has achieved prime ministerial predominance by mobilising the resources of government.

The American President whom Blair resembles most is not George W. Bush. To some extent, it is Bill Clinton as a communicator. The late Roy Jenkins saw similarities between Blair and Franklin D. Roosevelt. FDR demonstrated -and Blair also has -a great ability to enthuse those coming to see them for half an hour.

Their visitors "go out thinking that the world is a better place and they are a more important person than they were when they went in". However, "sometimes people misinterpret them because they give more agreement to the proposition of the other person than is really the case". Just ask Gordon Brown and Paddy Ashdown.

Yet FDR had, and Blair so far has had, an almost chameleon-like ability for reinvention. The original programme could be endlessly rewritten. What matters is a sense of confidence and direction. In his biographical essay on FDR (incomplete on his death last January, but finished by his friend Neustadt, and just published in America), Lord Jenkins of Hillhead argues that Roosevelt was "much more of an improviser than an ideolgue".

That is Blair's dilemma now. Can he reinvent himself? Long-serving leaders, such as FDR, Konrad Adenauer and Helmut Kohl in Germany, as well as Baroness Thatcher, all showed flexibility. They all faced serious political reverses as Mr Blair does now, but managed to pick themselves up, brush aside opposition and convey fresh momentum. In that sense, the political skills of a president and prime minister are similar. Yet, unlike a president, Mr Blair can never free himself from his domestic political constraints. His power, abroad as much as at home, is directly related to his ability to dominate public opinion and Parliament. No amount of effusive praise from George W. Bush will help him in the Commons division lobbies.

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