Labour members must be handed greater power to choose the party’s candidates and leadership after their role in securing a strong election result, one of Jeremy Corbyn’s closest allies says.

Jon Lansman, who founded the Momentum group that formed from Corbyn’s first leadership campaign, said the party needed to ensure that the thousands of members who had signed up after being enthused by the Labour leader did not drift away.

In an interview with the Observer, Lansman suggested that the case for empowering members was stronger than ever, backing proposals that would allow local parties to nominate leadership candidates and another that would make it easier for members to select a new Labour candidate.

“We have shown in this general election how mobilising enthusiastic people wanting to campaign for a party, believing in it, and being able to persuade people on the doorstep – how effective that can be,” he said.

“If you want to keep those people enthusiastic and committed, you have got to give them some ownership, you have got to give them some say in key decisions in the party – about policy, about who their representatives are.

“We have shown in the election campaign – in that fantastic turnaround – that the criticisms, that the pessimism about what Jeremy was offering and how it would resonate with people … they have been proved wrong. So I think democratising the party is essential.”

Labour activists in Glasgow hand out election material ahead of a stump speech by leader Jeremy Corbyn. Photograph: Murdo Macleod/The Observer

Momentum is not campaigning for any Labour rule changes as an organisation. However, Lansman is backing attempts by local parties to make a series of changes at next year’s conference (because the changes are being proposed by Labour branches, party rules mean they cannot be formally considered until 2018).

One would see branches handed the power to nominate leadership candidates – a power currently in the hands of MPs and MEPs. Such a move would ensure the party’s left always had a candidate in the running.

Another would ensure that sitting Labour MPs need the support of 66% of local party and trade union branches to be automatically reselected. Such a move would make it easier to reopen the selection process. Under the current rules, MPs need only a simple majority.

Some Labour MPs are deeply suspicious of the proposal, seeing it as a device to remove critics of the leadership. However, Lansman said that the concerns were misplaced. “Any hardworking MP who has good interpersonal relations – which any politician should have – with their members ought to have no fear from any selection process, even mandatory selection, though that’s not what [the left] are proposing.

It’s right that the members of the party, who finance it, who achieve the victories, have a role in choosing candidates

“It’s about involving members in choosing their candidates. I think it’s right that the members of the party, who finance it, who achieve the victories or play a large part in achieving the victories – there are of course other very important other factors – have a role in choosing their candidates and picking their policies.”

While the rule changes will be popular with many new members, much will depend on whether they gain significant support among trade unions, which still wield huge power at Labour’s conference.

Lansman argued that handing members more power would help the party avoid policy errors such as the Iraq invasion, which he said was symptomatic of a Labour leadership driven by an intolerance of dissent.

“There was a transformation of the Labour party through the New Labour years, which centralised the party enormously – in which there wasn’t any internal democracy,” he said. “That was true of all aspects of the party – policy-making, everyone had to be on message. It was all about message discipline. And the result of that was that there wasn’t just no challenge from the party membership to anything, no criticism or dissent permitted or genuine debate – there wasn’t even in the [parliamentary Labour party] or even in the cabinet.

“[That] is what led Chilcot [chairman of the Iraq war inquiry] to particularly point at the lack of challenge to really crucial policy decisions – which led to colossal mistakes and great damage to the party in the polls, ultimately.”

He said that allowing MPs the exclusive right to nominate leadership candidates almost led to Corbyn being blocked from running in 2015. “We very nearly didn’t have Jeremy on the ballot paper, which means, in my view, we would not have had that enormous surge of members coming into the party, we would not have had the result that we have just had in this election … Nobody has a monopoly of wisdom, including MPs.

“The important thing is that members, who elect the leader, have a broad choice that reflects the range of opinions in the party. I would want the Labour party to be a broad, pluralist, inclusive party. I would want there to be both wings, all sections of the party to be represented on the shortlist. It was right that Liz Kendall [seen as a Blairite] was on the shortlist, even though she only got 4.5% of the vote.”

Lansman also condemned online abuse after senior Labour figures, including Yvette Cooper, highlighted the attacks suffered by Labour MPs such as Luciana Berger at the hands of some Corbyn supporters. However, he criticised the Conservatives for running a personal campaign against Corbyn at the election.

“Luciana has been subject to the most appalling antisemitic abuse,” he said. “I think Diane Abbott suffers the most appalling abuse, I think women generally. And I think there is no place for that in politics. Jeremy’s first leadership campaign made very clear his views on abuse and personal attacks. We certainly would echo it. I would and Momentum would.

“But the biggest personal attacks in the election came from the Tories, who waged a campaign which was largely based on smearing our leader.”