David Miliband will this week warn Labour that it will not return to government until it wins back support across the class divide – and that it cannot retreat into its working-class heartlands following its May election defeat.
The Labour leadership front-runner will use a campaign speech to tell activists the party still has much to do to re-establish its electoral appeal after its support "collapsed across social classes" three months ago.
"You just can't craft an election majority out of a minority. It is dangerous to pretend we don't need the middle classes – just as it would be to suggest Labour does not need to win back the hope and trust of working-class voters.
"Too many people in both groups felt they had real reasons for no longer supporting Labour, and we must address them all."
The former foreign secretary's rallying-cry comes as he enters the final month before the leadership result on 25 September.
Ed Balls, who remains an outsider in the polls, last night tried to broaden his support by issuing a 10-point "contract with the Labour Party". He also launched a consultation on improving Labour's policy-making process "so it does not lose touch with the concerns of working people".
Mr Balls's document included pledges to increase party membership and strengthen links with the unions, to support greater representation of women and to end "undemocratic, imposed selections" of election candidates.
In a fundamental criticism of Labour's election campaign, he added: "On some policy areas – Iraq, tuition fees, agency workers, housing, and fair migration – we lost touch and lost our way."