Lord Mandelson said Ed Miliband has "way exceeded my expectations" and hailed the Labour leader's election campaign.
The former strategy chief and cabinet minister - who last month declined to positively endorse the party leader - said Miliband had confounded his critics and found the national "Zeitgeist".
"I started feeling at the end of last week that something had shifted," Mandelson told Channel 4 News.
"That the plates had moved and Labour has captured a Zeitgeist in this campaign and the most striking feature of it is what's happened to the two leaders.
"Miliband has moved forwards. He gained credibility. He's exceeded most people's expectations. I suppose in a sense mine as well. David Cameron on the other hand has moved backwards."
Asked in March whether the party would do better under another leader, the peer had stopped well short of an enthusiastic endorsement of Miliband.