No one – including John McCain, the senator from Arizona, who leads the field of Republican aspirants – could match the drawing power of the former president from Arkansas, and the 45-year-old Mr Obama who launched a book of canny political musings called The Audacity of Hope during the mid-term campaign.

Supporters of Hillary Clinton, who has yet to confirm her 2008 intentions, point out that she was busy campaigning for re-election to her New York Senate seat. Both her husband and Mr Obama, who was elected to a six-year term in 2004, were free to criss-cross America on behalf of others.

But Mrs Clinton’s New York race was always a shoo-in. And many Democrats, particularly south of the Mason-Dixon line in close contests such as the senatorial race in Tennessee, made it plain her presence would not be helpful.

The same was largely true in the north. Both Mr Clinton and Mr Obama visited the key swing state of Ohio several times, whereas Mrs Clinton did not visit once. Overall, Mr Clinton attended 100 events in 31 states, including 12 states in the last week alone. 

Supporters of Mrs Clinton, who still include most of the Democratic fundraising establishment, point out that Mr Clinton has been building up capital for his wife in states she has yet to visit, including Iowa, which stages key caucuses early in the race to nominate presidential candidates.

But few Democrats can disguise their excitement at Mr Obama’s meteoric rise. “Barack Obama is like a rock star – he is so fresh,” said Larry Sabato, head of the political centre at the University of Virginia. “Most of the rest of the field look like the Rolling Stones.”

Confirmation of Mr Obama’s rising stock came last week when a Fox News poll showed Mr McCain beating Mr Obama by only two points in 2008, compared with a five-point margin over Mrs Clinton. However, Mrs Clinton still retains a lead over Mr Obama among Democratic voters. Among the remaining Democratic potentials, John Edwards, the losing 2004 vice-presidential contender, is the most popular. His scepticism on globalisation could make him a formidable contender in the next two years.

The same cannot be said for the losing 2004 candidate, John Kerry, whose invitations suddenly vanished last week after he “botched” a joke in which he appeared to denigrate US troops serving in Iraq. In a rare flash of humour, Dick Cheney, the vice-president, reminded voters last week of Mr Kerry’s notorious “flip-flopping” in 2004 when he said: “Kerry was for the joke before he was against the joke.”

Most assume Mr Kerry’s chances to be little better than those of Mark Warner, the former governor of Virginia, who was once considered the greatest threat to Mrs Clinton’s candidature, but who last month suddenly withdrew his name from the 2008 race.

A handful of others, including Tom Vilsack, governor of Iowa, and Evan Bayh, senator for Indiana, have good credentials.

But few now doubt that Mrs Clinton’s real headache is Mr Obama – assuming he chooses to run. “Obama’s sudden prominence has dramatically changed the climate,” said Simon Rosenberg of the New Democratic Network. “He embodies the spirit of the times.”

Among Republicans, the biggest gainer has probably been Mitt Romney, the outgoing governor of Massachusetts, who benefited from the gaffe-prone campaign of George Allen, the Virginia Republican, previously seen as the best conservative Republican hope to defeat Mr McCain.

But Mr Romney’s Mormon faith is still a liability among socially conservative Republicans, particularly evangelicals, who make up a third of the party’s base. And although it appeals to independents, the healthcare plan Mr Romney pushed through in Massachusetts this year has made fiscally conservative Republicans suspicious.

“I find [Romney’s] healthcare bill troubling,” said Pat Toomey, president of the Club for Growth, an influential conservative group. Mr Toomey is even more suspicious of Mr McCain. “He’s got real problems with conservatives,” said Mr Toomey. “It’s not clear who is going to emerge as the conservative standard bearer.”

A popular campaign figure was Rudolph Giuliani, the former mayor of New York, whose response to the September 11 attacks won him national admiration. However, Mr Giuliani’s liberal stance on social issues could prove insuperable at the primaries in spite of the extensive use to which he has been put by embattled Republicans.

Meanwhile, like Mr Giuliani, Newt Gingrich, the former House of Representatives speaker, who spearheaded the dramatic 1994 Republican mid-term victory, has a history of marital difficulties that would hobble his campaign before it began in spite of his overt 2008 ambitions. Bill Frist, the retiring Republican Senate leader, was barely seen on the campaign trail having acquitted his leadership poorly on Capitol Hill.

Which leaves Mr McCain who, although he is 71, has clocked up almost 140,000 miles since early September. Although he is famously short-tempered, Mr McCain’s stock is higher with the public than any other aspirant – including Mr Obama.

Although he has yet to declare his candidacy, Mr McCain is the least coy. The former pilot, who was tortured in Vietnam, recently dismissed suggestions he should run for vice-president. “I have no intention of submitting myself to torture for a second time,” he told a private event of Republican donors in Ohio.