Call it the Potomac shuffle, the traditional election-year dance in which a candidate who has earlier moved left or right to win over the party faithful in a primary campaign promptly slides back to the centre to appeal to the rest of the country. Barack Obama, quite a mover on the dancefloor, has spent the month since he beat Hillary Clinton to the Democratic nomination giving a demonstration of this time-honoured piece of Washington choreography - and at an unusually high tempo, too.

Just yesterday he announced, in a speech on religion aimed at wooing evangelicals - who Democrats believe are no longer a guaranteed bloc for the Republicans - that he would continue George Bush's support for "faith-based initiatives", channelling public money to religious groups to perform social services, whether drug rehab or care for the poor. (Side note: watch for David Cameron, who also favours this approach, to claim he is Obama's spiritual brother.)

A day earlier Obama had delivered an equally long address on the virtues of patriotism. On his lapel was the flag pin he has worn since mid-May, the same pin he once disdained as an unnecessary, shallow display of love of country. More substantively, Obama has tacked towards the centre on a string of issues where a matter of months ago he was to be found much further left.

He once opposed legislation needed for Bush's much-reviled programme of domestic surveillance; now he supports a new law that would grant immunity to phone companies that help the government eavesdrop on US citizens. He was an advocate of gun control, but only hemmed and hawed when the supreme court struck down the District of Columbia's ban on hand guns last week. He now says he will consider joining his Republican opponent John McCain in calling for a cut in the corporate tax rate. Suffice to say, these were not positions Obama took when he was trying to win Democratic votes in New Hampshire or Iowa.

What will most strike - and disappoint - those outside the United States, those who have been all but panting in their excitement at the prospect of an Obama presidency as a break from the Bush era, is the Democrat's march rightward on foreign policy. His signature stance, which propelled him from obscurity into challenging the mighty Clinton, was his opposition to the Iraq war and his promise to bring the troops home. Now, though, the talk is not of immediate withdrawal but of pulling out one or two "combat" brigades a month, a pledge so vague it leaves acres of wriggle room. The Wall Street Journal reported last week that the Iraqi foreign minister, Hoshyar Zebari, had been concerned that Obama would withdraw US forces too hastily, but came away from a long phone call with the candidate "reassured" that a President Obama would not do anything too "drastic".

Last autumn Obama skilfully contrasted himself with Clinton by promising to replace Bush's bellicosity with a willingness to talk even to America's sworn enemies, the president of Iran among them. But in a major foreign policy speech last month, he diluted that promise of talks and said he would do "everything in my power" to stop Iran getting a nuclear bomb - pointedly repeating the word "everything" in case Tehran had not got the message.

Obama's most leftwing supporters have dutifully played the role expected of them, howling betrayal, with the activists of MoveOn leading the lamentation. But even centrist Democrats, who understand the politics of the Potomac shuffle only too well, are troubled. "He's overdoing it," says a longtime party strategist (who, admittedly, did not back Obama in the primaries). "He's reversing too many positions too quickly."

This view is shared by those who reckon Obama is a special case. Of course, they say, presidential candidates always have to shift towards the centre after a primary campaign; indeed, as Michael Tomasky argued in these pages on Monday, it is a sign of McCain's weakness that he has not been able to do so nearly as extensively as his opponent. But Obama is not just any candidate.

"You can't do it if you've run as Gandhi," says Leon Wieseltier, who observes Washington from his perch at the magazine New Republic. He contrasts Obama with the Bill Clinton of 1992, who also moved rightward once he had bagged the nomination. Both men offered to transcend the old categories of left and right, but Clinton did so by promising to be ideologically flexible. Obama's implicit promise is that he is above left and right, not because he is pragmatic so much as because he is morally good. In this context, says Wieseltier, U-turns are much less tolerable: "They compromise his radiance."

There are other contrasts. Bill Clinton could finesse shifts by wrapping them in the language of policy detail; Obama is the very opposite of a policy wonk. He operates at 30,000 feet, somewhere in the rhetorical stratosphere. While the Clinton of 1992 talked obsessively about the economy, ensuring hard-pressed voters knew he felt their pain, Obama has not yet persuaded Americans that he has the answer to current woes. One Democratic strategist thinks Obama should be talking about rising gas and food prices every single day, not making grand speeches about faith or patriotism.

Indeed, Obama's sheer eloquence, combined with the string of recent policy flip-flops, points to another worry many Democrats are beginning to voice about their nominee: that there might be a hollowness to him, an absence where there should be a clear core of belief (not that Bill Clinton had that either). When Ronald Reagan ran in 1980, it was clear that he relied on two ideological pillars: free-market economics and a fierce anti-communism. Obama-sceptics ask what the equivalents would be for the senator from Illinois. The harshest say his campaign has no theme beyond his own life story. They note that in his three years in the Senate, he found no time to author legislation - though he did manage to write a second volume of autobiography.

Yet none of these grumblers and doomsayers will go so far as to predict that Obama will lose in November. He remains ahead in national polls, while some analysts say a reshaping of the electoral map is possible: if black turnout rises, several southern states could be Obama's, while wins out west, in New Mexico, Colorado and Nevada could, by themselves, be enough to give the Democrats the White House. Obama fervour remains high, especially among the young. This weekend word emerged of a new fad on college campuses, as students, both male and female, adopt Obama's middle name of Hussein - the target for much xenophobic whispering - as their own. Think "I am Spartacus".

In this light, Obama's U-turns look different. They suggest that he is determined not to be just another principled loser - and the Democrats have had plenty of those. The clearest illustration came in Obama's most blatant reverse. He had promised to stay within the system of taxpayer-funded campaign finance, which would have obliged him to stick to an $85m spending limit. Once it became clear he could raise, and spend, many times that amount, he broke his pledge. Sure, it was unprincipled. But it suggested a man bent on winning and ruthless enough to make sure he does. That's the standard operating procedure for Republicans. For Democrats it takes some getting used to.

freedland@theguardian.com