David Miliband has this evening launched his new online campaign -- davidmiliband.net -- to build a "movement for change" and help him win the Labour leadership race.

Addressing a small group of observers in Westminster, the former foreign secretary gave a frank assessment of why Labour lost the election, saying the party needs to become a "movement" again. The website carries a "practical" look at why the election was lost. However, Miliband sounded a note of optimism, saying it was an "exciting time to be on the centre left of politics", and that an "interdependent" world called for "social justice".

Tom Harris, the Labour MP for Glasgow South, who was first to declare for David Miliband -- even before Miliband did -- said that "David is the candidate for the leadership who can win".

The website will enable ordinary members of the public to organise meetings and discussions to discuss political change. It will have a chat forum as well as regular blogs from Miliband.

The site has echoes of the innovative online campaign launched by Barack Obama. Users can donate to the Miliband campaign but also to the Labour Party, and a third of sums given to the drive will be donated by Miliband to Labour's fighting fund at the next general election.

The site will list non-Labour supporters of the campaign, as well as MPs and senior party supporters.

"The broader the base, the more likely you are to strengthen party membership," Miliband said.

When I asked him to expand on his admission that Labour was not enough of a "movement" at the last election, Miliband re-emphasised his point when he launched his campaign: that Labour now has a "responsibility" to become the progressive, centre-left movement that wins over Lib Dems who did not want to crown David Cameron

"They ran on a ticket to keep the Tories out," he said, "but they kept them in." He added that the party must be "pluralist".

Miliband's new slogan is: "Bringing Labour together -- leading Labour to power".