Mr McTernan has previously been a staunch critic of the Labour leader, having called on him to resign before June’s general election and once branding MPs who nominated him to get on to the initial leadership ballot “morons”.
But the former Director of Political Operations to Mr Blair said the left-wing leader has done a “great job” and that there is “no threat to his leadership”.
“The real test for Momentum and supporters of Jeremy Corbyn is can they can take their enthusiasm and make it into an election supporting victory?,” he told Sky News.
“My experience in my local area is, of course I don’t agree with Momentum members about every single thing in politics, but I’m a member of Momentum myself, I joined it because of the energy around that movement.”
The left-wing group combines traditional political campaigning with producing social media videos, which were widely credited as playing a major role in the Labour leader winning a substantial youth vote in June.
Mr McTernan added that the organisation had proven wrong critics who thought its members would not be active in the party, as they embark on training members in marginal constituencies over the next year.
“Many people who saw them join the Labour party to support Jeremy Corbyn in his leadership elections were sceptical that they would do anything more than pay their money and cast their ballot,” he added.
“It turns out there is lots of energy among Momentum members and they were a part of the successful Labour campaign in the last general election which won so many seats…
“It’s a worry that they’re so large and getting so well organised that they might become a party within a party, but when they do things like this or their big ideas conference, ‘the world transformed’, they’re adding something, some richness to the Labour party that’s not in any other political party in Britain or even actually in Europe at the moment.”