Film producer Lord Puttnam - who backed Mr Miliband's brother David in the 2010 leadership contest -predicted another hung parliament in 2015.
He said he expected the Labour leader to be seen as "prime ministerial material" by voters "given the state of the country and given the options available".
But he suggested the elder Miliband brother - who quit Parliament to lead an aid charity after losing out on the leadership - would have made life "tougher" for David Cameron and Nick Clegg.
"At the time, and I think it's still true, it was a considerable gift to the Tory party, to the coalition," he told BBC Radio 4.
"David, in the last two years, as a leader would have been a much more difficult proposition. The day-to-day contest in Parliament would have been a lot tougher for Cameron and Clegg."