John Rentoul: Obama, the most dominant force in British politics

Who is the dominant figure in British politics today? His name was invoked by the Prime Minister in his news conference on Monday. "I agree with him entirely," said Gordon Brown. He was praised as "brave" and "right" by David Cameron on Wednesday. Both hope to be photographed with him in London this week.

I speak of Barack Obama, a phenomenon of politics not just in America but across the English-speaking world and beyond. Plainly, he is an exceptional candidate, although, like the prophets, he is even more exceptional outside his own land. Opinion polls in the US have him only four percentage points ahead of John McCain. Over here, on the other hand, a poll last week among people who can't vote for either man found Obama trouncing McCain by a five-to-one margin.

No wonder Brown and Cameron hope that some of that magic can be transmitted simply by shaking hands in front of a camera. When it comes to celebrity endorsements in politics, Obama is Bill Clinton and Nelson Mandela rolled into one. With a cherry on top.

Not that Obama would be partisan in British politics. That is why Cameron can be confident of his own meeting with the miracle worker, even if the Tories won't risk making the mistake Brown's office made in February, when it announced a meeting with McCain just before McCain cancelled.

Nor are British politicians supposed to take sides in the democratic business of another country, although such was Brown's eagerness to make common cause with Obama that he came close to breaking that convention last week. He nearly claimed that Obama's platform for the presidency was copied from New Labour policies on health, education and employment. "I know that the Democratic Party have looked at the successes that we have had in these areas as they formulated their policies," he said.

To be fair, Brown did not bring up the subject of Obama at his news conference: he was asked repeated questions by journalists. Nor did Cameron have any intention of setting off a media fuss last week by endorsing Obama's view that there was a racial dimension to feckless fatherhood. In giving an interview to The Guardian, Cameron wanted to add to the pressure on Labour MPs, in their newspaper of choice, on the issue of parliamentary expenses. But he was also asked about Obama.

So Cameron said he agreed with him, just as Brown had done. This was riskier for Cameron than for Brown, because of the Tory party's recent history with issues of race. But it "worked out very well", said a Tory spin-doctor, happily. Cameron received favourable notices for saying, through the proxy of Obama, that too many young black men run away from responsibility for their children.

Obama holds sway over British politics in much more than the breezy rhetoric of hope, change and responsibility, however. His progress over the next 107 days and – if he wins – beyond will define how Britain deals with the legacy of Iraq, the most divisive foreign policy issue for a generation. That may seem a little parochial, because the choice between President Obama and President McCain will also define the legacy of the Iraq war (and the longer shadow of 9/11) for the whole world.

The key point here is that, as our ComRes poll confirms today, most people wish that Britain had never got involved in Iraq in the first place. Most people here, as in the US, believe that the invasion was a terrible mistake. But it is a logical error to say that, because the invasion was wrong, US and British troops should leave at once. That error has been built into Obama's campaign for the presidency.

If Obama is elected, it will mark the long-delayed unwinding of the Iraq decision, which both George Bush and Tony Blair survived in elections in 2004 and 2005. Yet many of those who admire Obama for his opposition to the war are likely to be disappointed.

Until he won the Democratic nomination, most criticism of Obama came from the right. They are still at it. Charles Krauthammer in The Washington Post last week mocked his thin record as a legislator and a law academic: "His most memorable work is a biography of his favourite subject: himself."

Since he clinched the nomination, however, he has ditched left-wing baggage like a balloonist desperately trying to gain altitude. He came out against gun control and in favour of the death penalty for child rape; became pro-Israel and a hawk on Iran. Now that, as several commentators have observed, is "change you can believe in".

His policy on Iraq is more difficult to drop over the side. He is standing by his plan to withdraw US troops in 16 months. Whatever you thought about the decision to invade, that makes no sense. As Dick Morris, my favourite analyst of US politics, put it: "Experience has taught Americans to expect the worst about Iraq. They're inclined to agree that, if we pull out, al-Qa'ida will move in."

There was a moment last month – it was when Susan Sarandon, the actress, said she might emigrate to Italy or Canada if McCain won – when it seemed essential to the sanity of America that Obama should lose.

But, no, it is more important that the daydream should be broken. The idea that there is some kind of clean, different, painless, perfect alternative to politics as usual is a distraction from taking difficult, compromised decisions in an imperfect world. If Obama lost, too many people around the world could continue to believe that if only America got out of whatever it is in, everything would be better.

I think McCain is right about Iraq – that the surge has been a success, and that eventual troop withdrawal should depend on that success continuing. But I think it is more important, for America and the world, that Obama should be the one who learns the truth of this the hard way.

In office, he would be forced to use his eloquence and his global popularity to make the case for what is left of the coalition to see its responsibilities to the Iraqis through. Many of his supporters, especially outside the US, would see it as a betrayal. I think it would be a necessary one, by which he could at last heal the suspicion of American power that provides so many around the world with easy excuses.

For rolling comment on the US election visit: independent.co.uk/campaign08