The Labour leadership contest remained too close to call with one poll suggesting Ed Miliband could snatch victory by a narrow margin.
A survey of Labour members and trade unionists by YouGov for the Sunday Times showed bookies' favourite David Miliband enjoying a narrow lead on first preferences.
But the pollster calculated that once lower-ranking candidates were eliminated and their second choices redistributed the younger of the two brothers would prevail.
Voting is under way in the race to succeed Gordon Brown with the winner to be announced on the eve of this month's party conference in Manchester.
David Miliband secured the endorsement of veteran left-winger Dennis Skinner - a valuable backer in combating Ed's more left-leaning campaign.
The shadow foreign secretary used an interview on Sunday to paint himself as the "unity candidate" and warned rivals could condemn the party to opposition for decades.
But although the YouGov poll gave him a 36% to 32% lead on first preferences, YouGov suggested that could be converted to a 51% to 49% win for his brother by the final round. The poll put Andy Burnham in third place with 12%, Ed Balls fourth on 11% and backbencher Diane Abbott trailing on nine.
Party members, trade unionists and the party's MPs and MEPs each control a third of the vote under the complex electoral college system employed by Labour.
In a dig at the frontrunner, MP Sadiq Khan, Ed Miliband's campaign agent, said the party did not want "more of the same".
"We have gained support and momentum every day of the campaign. We continue to fight for every vote and we are increasingly confident that we can win all three sections of the electoral college," he said.