Poor David Miliband. To be the favourite and the front-runner in an election campaign can be a poisoned chalice. Victory is far from guaranteed. Just ask Hillary Clinton, who lost out in the race for the Democratic presidential nomination to a junior senator from Illinois in 2008. Or Michael Portillo, defeated in 2001.

So far, in Labour's first leadership election in 16 years, the shadow foreign secretary has attracted more high-profile endorsements - and raised far more money - than any of his four rivals. Yet he has failed, over the course of this protracted, summer-long campaign, to "seal the deal" with the Labour Party's electorate.

“Shock Labour Party leadership poll gives lead to Ed Miliband," declared the headline on the front page of the Sunday Times on 12 September, just 48 hours after David's campaign had received an endorsement from the veteran left-winger Dennis Skinner. The You­Gov survey - of 1,011 party members and 718 members of Labour-affiliated trade unions - put Ed on 51 per cent and David on 49 per cent, once second-preference votes from the third-placed Andy Burnham, the fourth-placed Ed Balls and the fifth-placed Diane Abbott were redistributed. (Mili-D was 4 points ahead of Mili-E - 36 to 32 - on the basis of first-preference votes only.)

The D Miliband camp has been in a state of semi-panic since that Sunday. "It's going to be tight," concedes a spokeswoman. David himself described the YouGov poll as a "wake up call" on a BBC programme on which he and I appeared that weekend.

Pole position

So what should he do? Is it too late for a "game-changing" moment? "David has been running a classic 'inevitability' strategy," says an Ed Miliband aide. "The attitude seems to have been: 'You'd better vote for me because I'm going to win. Get on board.'"

That strategy now lies in tatters and, in recent days, the E Miliband camp has been using the poll findings to try to persuade uncommitted Labour MPs to throw their support behind Ed, not David. "The race is neck and neck," wrote YouGov's Anthony Wells on 13 September, before concluding: "but Ed Miliband is now in pole position".

My hunch has always been that Ed will win, if only by the narrowest of margins - and, in fact, much of the empirical evidence suggests that he is performing even better than the headline figures from YouGov suggest.

First, consider the issue of turnout. Of the 1,729 respondents to the YouGov survey, about four in ten had already cast their ballots and, as the polling expert Mike Smithson has noted: "Of these, in both sections [party members and affiliated trade unionists], Ed Miliband was doing far better than among the samples as a whole." YouGov may have treated "have voted", "definitely will vote" and "will probably vote" as of equal value, but when the crucial "have voted" category is examined on its own, Ed leads his brother in first preferences, too, and not just second preferences.

Second, consider the role of the MPs and, in particular, their second preferences. In Labour's tripartite electoral college system of party members, affiliated trade unions and MPs and MEPs, it is the last section that wields the most influence: the vote of one MP is worth the votes of approximately 600 party members or 13,000 affiliated members.

YouGov used research conducted by the website Left Foot Forward to establish MPs' preferences but, as Wells acknowledges: "One big caveat is MPs' second preferences - there is little good information on how MPs will cast their second preferences." He says that the organisation "made the crude assumption that the second preferences of MPs who back Abbott, Balls and Burnham will divide evenly between David and Ed Miliband". This skews the result. Is it credible, for instance, to believe that Diane Abbott's left-wing supporters will split evenly between Ed and David?

“It is too simplistic to assume a 50:50 split in MPs' second preferences between David and Ed," a close ally of the younger Miliband tells me. "The real reason why we are now so confident and optimistic about victory is that our support among MPs, in terms of second preferences, is disproportionately high."

This late "surge" from Ed Miliband has spooked both his leadership rivals and the right-wing press. The Daily Mail and the Sun have already dubbed him "Red Ed", while supporters of David Miliband were overheard last week comparing his younger brother to Forrest Gump, the rather slow and gormless movie character played by Tom Hanks in 1994. Other allies of Mili-D have dismissed Mili-E as "Labour's Iain Duncan Smith".

Nothing but the truth

If that was not enough, the younger Miliband's honesty has also been called into question by his rivals - especially over the issue of the Iraq invasion, which the shadow energy secretary has described as a "profound mistake" and claimed to have opposed in private. But his brother, David, has stated: "Diane Abbott is the only candidate that can say she was against the war at the time." Ed Balls, too, has said it is "ridiculous" for Ed Miliband to claim he was privately anti-war in 2003. "He says he didn't support the war but I'm not sure I believe him," says a well-connected Labour source, who has decided to back David over Ed.

However, a close friend and former colleague of Ed Miliband tells me that he has no doubt whatsoever that the shadow energy secretary opposed the invasion of Iraq in March 2003. "I know for a fact that he was against the war because it was he who persuaded me of the merits of the anti-war case," says the friend. "I remember flying out to Cambridge [Massachusetts], where he was on a sabbatical lecturing at Harvard, and he argued very strongly that the UN weapons inspectors should be given more time to finish their work."

I have learned that Miliband Jr rang Gordon Brown from the United States to persuade the then chancellor of the exchequer to resist the drumbeat for war coming from inside No 10.

A former Downing Street aide says that Brown "took Ed's phone call very seriously but, ultimately, other views prevailed".

Ed M's position on Iraq has helped broaden his appeal across the party and has given him much-needed momentum. But what's this? The moment he becomes the "front-runner", a new ComRes poll emerges suggesting Labour voters prefer his brother by a two-to-one margin. The next week will be fraught.