He has been called too arrogant, too remote and too clever by half, but last week Senator Barack Obama was hit below the belt with a cruel new allegation: he may be too skinny to win the White House.

Suggestions that Obama’s slim physique is a liability in a nation of mostly overweight voters marked a dangerous new turn for the Democratic contender’s suddenly vulnerable presidential campaign.

The rapturous reception bestowed on Obama by awestruck Europeans on his recent world tour has given way to a barrage of Republican mockery and sly innuendo that has wiped out his lead in the opinion polls and turned his stately march towards the Democratic convention at the end of this month into a mud-slinging scramble.

Two new opinion polls on Friday showed that John McCain, the ageing Republican senator who had seemed to be lurching from pitfall to pothole in the wake of the Obama global parade, has not only caught up with his glamorous competitor — Gallup put them level at 44% each — but has also hit upon a damaging new strategy of vicious but effective personal attack.

“This campaign was always going to be closer than many people thought it would be,” said one jubilant Republican strategist last week. “Poor Obama seems to have thought it wouldn’t be close at all.”

Perhaps most menacingly for the man trying to become America’s first black president, the fawning US media coverage of his visits to London, Baghdad, Berlin and elsewhere is rapidly giving way to a more barbed approach.

Last week The Wall Street Journal suggested that Obama might be too thin and too fit to appeal to voters who tend to like candidates with flaws that they can identify with. Several analysts noted that widely circulated pictures of a red-faced Bill Clinton staggering into McDonald’s after a short jog did the former president no harm at all; millions of Americans knew just how he felt.

Obama’s enthusiasm for exercise first raised eyebrows last month, when he stopped three times in one day for workouts at Chicago gyms, prompting an Associated Press reporter to wonder: “Sometimes it’s hard to tell if Barack Obama is running for president or Mr Universe.”

It has also been widely noted that Obama sometimes seems appalled when presented at election meetings with the pride of local cuisine, often a fat-smothered hunk of meat or a sugary bun dripping in aerosol cream.

While most candidates tend to tuck in fearlessly, thereby assuring themselves positive coverage in the local paper, Obama once visited a chocolate factory in Pennsylvania but turned down a piece of cake on the grounds that it was “too decadent for me”. He lost the Pennsylvania primary and appears to have learnt his lesson: last week he was overheard asking for “pie” at a diner in Missouri.

On Friday the Journal wondered if Obama might suffer from his skinniness in potential swing states such as Georgia and Tennessee, which have more overweight people than the northern states more favourable to the Democrats.

Government statistics indicate that two-thirds of the overall voting population is overweight and almost a third is obese. Yet the 6ft 1in senator is reckoned to weigh 10lb-20lb less than the 190lb recommended weight for his height.

The notion that Obama is too thin to win was derided by many of his supporters, but the issue underlined a disturbing reality for the Democratic candidate. Polls have consistently shown that McCain is beating him among significant groups of voters, notably working-class white males and older suburban women, who complain he appears elitist or out of touch with average Americans.

Obama now appears vulnerable to the kind of character assassination that helped to do in Senator John Kerry, the Democratic contender in 2004. Kerry was successfully depicted by Republicans as an effete, windsurfing, brie-eating, French-speaking fop.

While Obama’s world tour was undeniably a diplomatic and personal triumph, its main effect domestically was to spur McCain into a negative political onslaught.

The 71-year-old Republican’s decision to launch a series of television attack advertisements mocking Obama’s celebrity, comparing him with Hollywood brats such as Britney Spears and Lindsay Lohan and disparaging his “divine” demeanour was a calculated risk: McCain had previously prided himself on his political character and had pledged a clean campaign.

Some Republicans warned that McCain was squandering his image of decency and risked alienating a key group of voters whom he still hopes to woo away from Obama: disillusioned former supporters of Hillary Clinton. John Weaver, a former top McCain aide, denounced the television ads as “childish” and “tomfoolery”.

Other analysts took one look at the daily tracking polls showing McCain snapping at Obama’s heels and predicted a summer of nonstop Republican abuse.

There were signs last week that the tension is getting to Obama, who in a rare misstep unwisely lashed out at Republicans he claimed were trying to scare off voters because he is “not like all those other presidents on the dollar bills”.

The overt racial reference to the white faces on US currency played straight into the hands of McCain, who promptly accused Obama of “playing the race card” — a cardinal sin that traditionally repels American voters frightened of racial confrontation.

The sour turn to the race raised the stakes for the next big announcements of the campaign: the vice-presidential choices. What should have been a straightforward selection for Obama, who was widely expected to name a safe but boring white man as his running mate, has been complicated by what is rapidly becoming his woman problem.

According to a recent Fox News survey, McCain is making significant inroads among women voters over 40, some of whom supported Hillary Clinton, whose admirers have all but given up hope that Obama might choose their idol as his running mate.

Two former Clinton aides last week closed down a Hillary-for-VP website called Voteboth.com. They were then stunned to learn that Obama was seriously considering two other women candidates: Governor Kathleen Sebelius of Kansas and Senator Claire McCaskill of Missouri.

Lanny Davis, a close friend of Hillary and former aide to President Bill Clinton, expressed outrage that Obama should consider any woman but Hillary.

“If anyone thinks that picking a woman will simply placate Hillary Clinton’s supporters, I think that’s very patronising,” he said.

McCain’s shortlist also includes two well known women: Governor Sarah Palin of Alaska and Carly Fiorina, former chief executive of Hewlett-Packard, the computer firm.

Grover Norquist, an influential conservative Republican, declared Palin “a good choice” and other analysts noted that if McCain selected a female running mate, there would be nothing thin about Obama’s woman problem.

McCain's celebrity attack ad

Explore World News

• World Agenda

• Iraq News

• US & Americas News

• Europe News

• Middle East News

• Afghanistan News

• Asia News

• Africa News

• Ireland News

Times Recommends

• 'New York has lost its edge'
• Copenhagen will fail – and quite right too
• More than 17,000 new underwater species found