Few people in Labour have heard of Michael Chessum, the self-described “hard-left, hard-remain” campaigner, who concedes he is an unusual frontman for a campaign that could transform the Labour party’s Brexit policy.

“Last year the faces were Chuka Umunna, Heidi Alexander, I can’t really believe that this year, it’s people like me, it’s weird,” he says in his shared south London office, surrounded by anti-Brexit paraphernalia and protest signs.

Chessum’s group, Another Europe is Possible, is just one of a number of grassroots organisations that have forced the party to have a conversation about Brexit that it has long delayed.

More than 150 constituency Labour parties have submitted Brexit motions for debate at the party’s conference, which begins in Liverpool on Sunday, with the vast majority calling for a vote on the final deal.

It is the result of an intense and unprecedented ground campaign by a coalition of grassroots groups. One Labour official said the number of motions on Brexit was so vast that they could have difficulty devising other motions for debate on other topics.

Last year in Brighton, delegates followed Momentum’s direction to save the leadership’s blushes and blocked any potential vote on Brexit. In Liverpool, delegates will arrive with an unprecedented mandate from members to put a second referendum on the table after a campaign led by the pro-Corbyn grassroots.

“The numbers are completely crazy. I thought we’d get 50,” one pro-remain MP said.

Many campaigners and MPs hope the efforts will resonate beyond the party conference in ways that could have a major and unforeseen effect on the parliamentary arithmetic.

“It gives MPs some cover,” another MP said. “Some have resisted voting on anti-Brexit amendments because they will get grief locally for seeming to be disloyal. But if their CLP has put this motion forward, it gives them the freedom to vote with their conscience.”

Chessum, a member of Momentum’s first steering committee, said he wanted to transform the pro-remain cause within the party into one that was explicitly leftwing, tribally Labour and pro-Corbyn.

“I am a diehard remainer and a diehard member of the hard left,” he said. “There is no bespoke Labour Brexit on the table. There is no time, even with a general election.”

Chessum argues he is giving the leadership space to change its position, rather than closing it into a corner. “The Labour right loves ‘Corbyn v the membership’. That’s not what this is.”

The group is technically cross-party and has worked with other leftwingers and Greens. For the summer, it has been working in co-ordination and hosting phone banks for another explicitly Labour group run by Mike Buckley, a former aid worker and community organiser, who founded Labour for a People’s Vote this year.

Buckley said he found a huge gap in the Labour party for a group that was pro-Corbyn and anti-Brexit, claiming that it represented about 70% of the membership.

“There was nothing to rally behind,” he said. “The people talking publicly about having another referendum, however well intentioned they are, they are not going to gather the majority of Labour party members behind them because they are seen as being anti-Jeremy.”

Both groups have regional organisers and have relied on sign-ups and their existing networks, in local parties and Momentum groups, some relics of the 2016 referendum campaign. Among the organisers is Alana Ivanova, a Momentum campaigner who started a petition to get the grassroots group to adopt a second referendum.

Ivanova said campaigners hoped to tap into new Labour members’ passion for party democracy. “Brexit is a major test for that,” she said.

Ana Oppenheim, another organiser, said many members did not know how to use the party’s existing democratic levers. “Now the left’s in control, it’s a chance to use them for something we believe in,” she said.

For all the groups involved, there is no roadmap. The summer’s efforts have involved a painstaking task of identifying pro-European members in more than 600 local parties, through a mix of local contacts, friends of friends, and online signups, and getting them to champion a motion through their CLP.

Buckley said that organising local parties has been “surprisingly difficult” and relied on local knowledge.

Both groups have links with the main cross-party campaign People’s Vote, but maintain a public separation. Another mainstream group, Best for Britain, the anti-Brexit campaign that has the financial backing of billionaire philanthropist George Soros, has given £70,000 to Another Europe is Possible.

Pro-European MPs have aided the groups in the background, helping them make connections, but have deliberately not led the campaigning.

Though the groups are mainly run by young activists, it is older Labour members who have been the focus of some of the summer campaigning, because they make up the bulk of new members active in CLPs.

The group that has most successfully used this tactic is the energetic pro-remain youth group For Future’s Sake (FFS) – a cross-party campaign which has close links to the official People’s Vote movement.

Its young organisers have made provocative videos, deliberately designed to appeal to older members, placing the arguments about Brexit in the context of progressive political movements in British history, including the campaign for women’s suffrage and fighting the poll tax.

FFS’s Liron Velleman has been co-ordinating its campaign on the ground to get CLPs to submit motions. “We get young people to sign up and then encourage older people from the CLP to get involved in the fight,” he said. “We tell them, our generation needs yours.”

Previously unreleased data showed the average age of Labour party members and supporters who joined after 2015 is 51, and the most common age of this cohort to be 66. The project’s summer 2017 survey of Labour members revealed an average age of 53, with the most common age being 70.

Professor Tim Bale, who undertook the study as part of the Party Members Project at Queen Mary University of London and Sussex University, also reveals that older members are actually more pro-European than their younger comrades. His 2017 survey found 54% of members aged over 60 would definitely back a second referendum, compared with 31% of 18 to 24 year olds.

Now the local party organisation is over, campaigners from the main groups are turning their attention to the party conference itself. The first hurdle is ensuring Brexit is a topic on the agenda at conference, as one of eight topics chosen by members.

Last year, Momentum steered its delegates to vote for other topics, including housing, NHS and the railways. That will not be repeated, its national co-ordinator, Laura Parker, said. “Last year, the context was very different. Now, it’s absolutely inevitable there will be a discussion on conference floor, I can’t conceive there won’t be – we’re 200 days away,” she said. “Without a doubt, there has to be a debate.”

Momentum has not backed a new referendum and there is clearly some caution about doing so. Parker’s instincts are pro-Europe, but she is wary of restricting the Labour leadership’s political flexibility.“We are waiting to see what motions come to conference floor,” she said.

Once Brexit is confirmed as a topic for debate in the ballot, delegates from those CLPs will meet trade union representatives and Labour’s Brexit secretary Keir Starmer on Sunday night to thrash out a compromise motion that could go to a vote.

Amanda Chetwynd-Cowieson, an FFS co-founder, said the groups would be co-ordinating. “The one cast-iron agreement across all the groups is that a people’s vote is the way to solve the crisis,” she said.

Some senior Labour sources cautioned that it was now uncertain if delegates would be responsive to either Momentum or any of the other campaign groups.

“Delegates are totally unbiddable now,” one said. “Last year, there was some degree of predictability that most delegates would at least do what Momentum instructed, but that is no longer the case.”

Key to the success of any motion or compromise is the support of trade unions. Campaigners have been closely watching both the Unite conference and the subsequent Trades Unions Congress, where union leaders accepted or put forward motions that could be a blueprint for the Labour outcome.

One of the “big three” unions, GMB, has already officially backed a new referendum, after polling its members. Smaller unions have also been key players, most notably TSSA and its general secretary, Manuel Cortes, and Community, a centrist union with many Brexit-voting members whose industrial membership recently backed a vote on the deal.

Unite, Labour’s most generous donor and the UK’s second largest union, left the door open to backing a second referendum at its conference, with a leadership statement saying it was “open to the possibility of a popular vote being held on any deal, depending on political circumstances”.

That statement is now the most likely model for a Labour compromise. “The key voice in the room will be Unite,” one senior union source said.

The third major union, Unison, the largest in the UK, has remained studiously neutral. “They will be in the room at conference, they might act as honest brokers, but they don’t want to be forced to take a position,” the source said.

The final outcome may ultimately be a fudge, the party’s governing national executive committee could offer a policy statement, keeping a referendum on the table without endorsing it. Starmer and the shadow chancellor, John McDonnell, have trialled that line in recent months.

However, even if the NEC offers a statement, delegates could still push for an additional vote and a more radical motion.

In the end, there is little will at the top of the party to prevent a Brexit debate taking place. “We would look ridiculous,” one source close to the shadow cabinet said. “The headlines last year were bad, but this would look terrible to outsiders watching.”