Labour leadership contender David Miliband will claim that only attracting a new "coalition of voters" from across the political spectrum will result in electoral success.
Mr Miliband will use a major speech, billed by aides as the most important in his campaign, to promise an approach of "realistic radicalism" if he succeeds in the contest.
The shadow foreign secretary, who has the support of 100 Labour MPs, will warn against retreating to the party's left-wing comfort zone.
Mr Miliband will say: "The decision of the Lib Dems to join a Conservative Government creates a big opportunity for the Labour Party to realign the centre left of British politics.
"But for me, that's not enough. I see the primary task for Labour as shifting the centre ground of British politics.
"And that means more than an agenda for changing Labour. It demands an agenda for changing Britain."
He will add: "To win again we need working class voters, middle class voters, Conservative voters and Lib Dem voters as we drive the Tories out of power."
Mr Miliband will say the party must "look forward for new ideas and outwards for a new coalition of voters".
"There is no future for Labour in the comfortable but deadening policies of the past.
"And there is no future in a politics based on a tactical, patchwork approach to building electoral support."