Barack's brief visit to London coincided with the opening of the new Batman film in British cinemas. How fitting. On his journey through Europe last week the US presidential contender garnered adulation fit for a superhero. As a series of polls has shown, if the British public could vote on the American race,Obama would crush his rival John McCain by a margin of four or five to one.

Even 28 Tory MPs are backing him amidst an outbreak of Westminster Obamamania, despite his left-wing voting record, which made him the most liberal senator of 2007.

The British and European euphoria surrounding Obama's trip is easy to understand. He is widely perceived as a charismatic, JFK-like figure, offering a seductive vision of a softer, sensitive America, while pledging to transform the negative image of the most powerful nation on earth. But let's consider the British national interest. When you push the spin and hype aside, there is scant evidence to suggest that an Obama presidency would actually strengthen the Anglo-American alliance, which has long been the engine of the free world. In fact, there is a risk it would be significantly weakened.

The special relationship was forged in the dark days of World War Two through the partnership between Winston Churchill and Franklin D Roosevelt, and has been a dominant feature of US and British foreign policy ever since. It is embodied in the close-knit military and intelligence ties between the two nations, as well as the huge volume of mutual trade and investment.

The alliance reached its zenith in the 1980s with the leadership of Margaret Thatcher and Ronald Reagan, a powerful partnership that successfully faced down the Soviet empire. After a period of decline it was successfully revived by George W Bush and Tony Blair in the wake of the 9/11 terrorist attacks.

The Republican political elites in Washington still see the special relationship as critically important, a view shared by Senator McCain. In contrast, the Democrats have shown little affection for Britain in recent years, partly because of their love affair with continental Europe, but also due to Blair's unstinting support for Bush over the war in Iraq.

After meeting with Gordon Brown yesterday, Senator Obama briefly acknowledged the special relationship, but only after prompting by journalists. He has yet to show much affinity with Britain; we should be wary of taking yesterday's remarks at face value. In fact, his key foreign policy speeches have not mentioned Britain at all.

An Obama presidency could well usher in a seismic shift in the transatlantic alliance, with the centre of gravity of US policy in Europe moving away from London towards Berlin and Paris. His controversial decision to make Germany and France the centrepiece of his European tour, with Britain tacked on almost as an afterthought, is a clear sign of the importance placed by the Obama camp on enhanced relations with the continent.

The Democrats are traditionally strong supporters of a united, federal Europe, and have little sympathy for British objections to the further centralisation of political, military and economic power in Brussels. If David Cameron becomes prime minister at the head of a Eurosceptic administration, Downing Street and an Obama White House would be worlds apart.

And it won't only be Obama's European policy that will grate in London. Closely aligning himself with powerful labour unions, Obama has pronounced protectionist tendencies. He has opposed the North American Free Trade Agreement as well as a host of free trade treaties from Colombia to South Korea.

If the Left wins control of the White House in November's election, many fear a new era of American protectionism will emerge. This could have a real and tangible impact in the UK - in shielding domestic US workers from foreign competition, including in the sensitive area of defence contracts, the Democrats would put thousands of British jobs at stake. This will draw the White House into conflict with the pro-free trade UK.

In his support for protectionist measures against the effects of globalisation, Obama will find common ground with several European leaders, including the French president, Nicolas Sarkozy, and it will consequently become harder for Britain to break down Europe's own protectionist follies such as the Common Agricultural Policy.

Britain's chattering classes should be careful what they wish for. Senator Obama promises change and a bold new course for the United States. The end result may be an America that looks away from Britain and erects higher barriers to trade and investment.

The special relationship has lasted over 60 years as the most powerful and successful partnership of modern times. Unfortunately it might not survive the next presidential election.

Nile Gardiner is director of the Margaret Thatcher Centre for Freedom at the Heritage Foundation in Washington, DC.