David Miliband will officially launch his campaign for the Labour leadership today with an appeal to the idealism of the party's supporters.
Mr Miliband and his brother Ed are the only candidates so far to throw their hats into the ring in the contest to replace Gordon Brown, but more contenders are expected to declare this week.
The former foreign secretary is the bookies' favourite for the leadership, with two polls yesterday suggesting he had the support of 32% of the public - significantly more than any rival.
But Labour's complicated electoral college system means that support among MPs, grassroots members and trade unions counts for more than popularity with the general public.
Mr Miliband has said he will tour the country during the leadership campaign to revive Labour's "conversation" with voters.
Launching his campaign in his South Shields constituency today, he will say: "Labour lost the election because our conversation with the public broke down.
"We need to restart it with our most precious asset - idealism for a better future.
"Idealism is the lifeblood of our party. I am an idealist. I believe injustice is real but not inevitable, and it is the job of politics to attack it, at home and abroad.
"I believe that you judge a country by the condition of the weak not just the strong, that you build strong communities on responsibilities as well as rights, that we are joined by humanity and self interest with people around the globe.
"I believe we need a market economy but not a market society. These are values that it is our duty to nurture: justice, compassion, sustainability.
"I am convinced that these beliefs are more relevant now than ever. The job of the next leader is to show how they can change our country."
Mr Miliband faces a tough challenge for the leadership from his younger brother Ed, the former energy secretary, who today won the support of former Cabinet colleague Peter Hain.
Mr Hain said: "Ed offers the fresh start Labour needs. He is a dynamic leader for the 21st century who will help us renew, learn the lessons from our defeat and energise our party so that we are connected to and active in our communities.
"To win Labour must consign Blairite/Brownite factionalism to the dustbin, and Ed Miliband is best able to do that. He has shown that he can be the bridge to unite the party behind a future vision of social justice, equal opportunities, redistribution of power, and green policies.
"This is a new era for our country, but also our party. Simply regurgitating 1990s New Labour mantras will fail, as will lurching back to 1980s Old Labour dogma. Under Ed's leadership, Labour can rediscover its soul and its ability to inspire modern Britain again, as we fight to win the next election."