In an aggressive campaign against the Labour leader, the Tories have released the findings of a private poll by Populus which shows Miliband is seen as weak and indecisive.
Nearly 65% of Labour voters believe David Miliband would have made a better leader than his younger brother, according to the poll and more than half of Labour voters, 56%, believe the wrong brother was elected leader. This rises to 67% among overall voters. Most (73%) voters believe Ed Miliband does not have the qualities required of a prime minister in tough economic times. This view is shared by nearly half (46%) of Labour voters.
A change of leadership might lead to an improvement for Labour which has a solid lead over the Tories in opinion polls but whose leader is behind David Cameron on strength and economic credibility. The poll found 51% of voters (and 64% of swing voters) said they would be more likely to vote Labour if the party had a stronger leader.
The poll found 25% of voters and 56% of Labour voters believe he is "trusted to repair Britain's finances".
The Tories will step up their assault on Miliband at the start of the Labour conference in Manchester by driving a poster round the city which will say: "Labour isn't learning." The poster, inspired by the Tories' "Labour isn't working" poster during Margaret Thatcher's 1979 election campaign, features pictures of Ed Miliband and Ed Balls in a classroom.
It is designed to show that Labour has not learnt any lessons from its time in office, when it ran up a record peacetime fiscal deficit, because the party is still committed to unsustainable levels of borrowing.
Cameron used the interview by Balls in Friday's Guardian, in which the shadow chancellor announced that he would hold a "zero-based" spending review if Labour wins the next election, to highlight this thinking.
In Brazil, the prime minister said: "What Labour are basically saying is: 'Elect us to government and then we will have a spending review…We messed up the economy, we got you into debt, let us back into power and we will think about it.' It is an absurd notion."
The Tories tried to highlight Miliband's weaknesses by asking Populus to put a series of negative statements about the Labour leader, originally voiced by others, to voters. Populus, founded by the Downing Street strategy director Andrew Cooper who commissioned the poll, interviewed just over 2,000 voters.
The poll found 72% of all voters believe that Miliband is "just too weak to be a credible prime minister". This view is held by 47% of Labour voters.
Only 20% of voters believe Miliband has prime ministerial qualities. Fewer than half (44%) of Labour voters believe he has such qualities.
Miliband enjoys even worse support when voters were asked if he is charismatic. Just 18% of voters overall and 34% of Labour voters believe he is charismatic.
The release of the poll follows a debate at the highest levels of the Conservative party about how best to attack Miliband. The group examined two key lines of attack: that Labour has learnt nothing from its time in office, as it seeks to increase borrowing, or Labour is not credible because it has not yet set out any firm policies on a range of areas from health to education.
George Osborne, who had always favoured focusing on Labour's failure to learn lessons, gave serious consideration to the second line of attack. But in the end the chancellor stuck with his instincts. The key members of the group were Osborne, Cooper and the No 10 director of communications, Craig Oliver.
A Conservative spokesman said: "This poll reveals that, though Labour have a headline lead in the poll, his leadership characteristics are incredibly poor. So the reality is: can he sustain that lead?"
Populus interviewed 2,061 adults online between 26-28 September in Great Britain. The results were weighted to be representative of the whole population. The poll was commissioned by the Conservative party.
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image: © EdMiliband