As if he had any other choice, David Miliband has said he "passionately" wants Labour to win the general election and said his brother Ed would make a good prime minister.
The former foreign secretary, who lost out in the party's leadership contest to Ed, said his brother has "the clarity, the vision, the determination".
In an interview with the Financial Times, he was asked who he thinks is going to win the 2015 general election, and he said: "I passionately want Labour to win - and Ed to win."
Asked if his brother would make a good prime minister, he told the paper: "Of course. I would know that better than most."
Questioned about his brother's qualities, Miliband said: "What I would say is that the clarity, the vision, the determination, those are all important qualities."
Ed Miliband has endured an intense bout of speculation about his future recently, with deep unrest among the ranks of backbench MPs and claims that some of his top team are plotting against him.
Last month, Tony Blair said he thought Miliband is ''robust enough'' to deal with the swirling unrest over his leadership, and offered his ''full support''.
Meanwhile, Miliband insisted he has the ''resilience'' to cope with the criticism he faced and declared that Labour's beliefs will see the party win the general election.
David Miliband is the president and CEO of the International Rescue Committee and is based in New York.
Miliband did not rule out a return to UK politics, telling the newspaper that he did not intend to remain forever in the US or take citizenship there.
"You just don't know, do you?", he said, in reference to the fact that his own career had already taken one unexpected turn.
"Tony Blair and John Major have said that they wish they'd done their post-premiership jobs before they became prime minister," he noted
Asked if he saw his IRC position as such a preparation for Number 10, he said: "That's not the way I conceived it.
"I miss my friends, my neighbourhood, my colleagues, obviously. But I am absolutely sure this is the right place for me and my family to be at this moment," he said.
"I'm doing something that really speaks to my values and my passions."
He made a fresh appeal to pro-European forces in the UK to make the case more strongly but said he did not expect the British public would be so "unbelievably stupid" to vote to leave the EU in a referendum.
"Those on the pro-British - as I call it - pro-European side of the argument have got to make the case that we get far more from being at the table than shouting with a loud hailer outside the room.
"I have this residual faith in the common sense of the British people that generally they don't do stupid things. And it would be unbelievably stupid to walk out of the European Union."