Leftwing pressure group Momentum is asking Labour parliamentary contenders to sign a contract that ties them to the “political objectives” set out in the organisation’s constitution to secure its support in upcoming selection battles, the Observer has learned.

Several contenders to be Labour candidates in marginal seats are understood to have signed the contract. The 13-point “political accord for Momentum-backed candidates” asks candidates to “work to ensure the Labour manifesto (subject to future policy development) is fully implemented once Labour are in government”.

Included in the signed contract is the commitment to “revitalise the Labour party by building on the values, energy and enthusiasm of the Jeremy for Leader campaign”. Some Labour MPs are known to be alarmed by the move, following an attempt by Labour activists to unseat the leader of Haringey council last month. Claire Kober fought off a challenge to win reselection in her ward, but it was seen as a warning that Momentum plans to support candidates that back Jeremy Corbyn at all levels of the party.

One Labour MP, who asked to remain anonymous, said: “It reflects a Stalinist approach to politics that Momentum would come up with such a contract for candidates. It has worrying implications for our democracy that there could be MPs in parliament who have signed away their right to independent judgment,” he said.

A spokesman for Momentum said the accord was drafted as part of efforts to improve the party’s ethics and to get candidates to take part in a “fair, honest debate on policies” and not personal attacks or harrassment.

“It just makes the commitment to higher ethical standards more explicit,” he added. “You want to back candidates who are good people – who support the code of ethics. We don’t have a programme. We are a campaigning organisation. The accord is more a version of what every Labour member signs up to. It just spells it out a bit more.”

Momentum chair Jon Lansman wants rank-and-file members to have more say over Labour policy. Photograph: Matt Crossick/Empics Entertainment

One candidate who has put his digital signature on the accord is Peter Chowney, leader of Hastings borough council. He said he was unconcerned by criticism that he had signed away his independence. Chowney was the Labour candidate in Hastings and Rye in the recent general election and came within 300 votes of unseating the home secretary, Amber Rudd.

“I’m not a member of Momentum, but they approached me and because I fully support the manifesto and was one of the few Labour council leaders to back Jeremy Corbyn in both his leadership contests, I am happy to sign up. It’s not like I am signing anything in blood,” he said.

The chair of Momentum, Jon Lansman, this year forced through a constitutional change to ensure all Momentum members were also Labour members, after concerns that the organisation was an entryist vehicle for expelled former party members. More recently Lansman, who is running for a place on Labour’s powerful ruling body, its national executive committee (NEC), has pressed the party to hand more power to Labour members.

Separately, he told the Observer that he would push to make it even easier for leftwing MPs to run for leader and for party members to select a different parliamentary candidate. The proposals amount to a new wave of party rule changes that would consolidate the power of its pro-Corbyn wing.

Lansman is running to win one of the new seats on the NEC that were created this year. He said that he wanted Labour to adopt digital tools, similar to those used by the Taiwanese government, to find the policy ideas that enjoy large support among Labour’s increasingly leftwing membership. The tool is soon to be trialled by Momentum, and allows members to agree or disagree with stated policy proposals. “For me, the biggest thing is giving members more influence over policymaking,” Lansman said. “There are now digital tools available which can capture people’s opinions. It can show popularity of a proposal.”

Asked if such an initiative could create pressure to reconsider Labour’s support for the Trident nuclear deterrent, he said there should be a “range of options” handed to the party’s annual conference.

“Personally, I am in favour of abolishing nuclear weapons and looking for alternatives,” Lansman said. “At the end of the day, Labour conference is the place that does bring together all sections of the Labour movement, including the unions.

“On defence, I would want conference to be given a choice, and it may be that what I want or members want doesn’t get a majority. I wouldn’t like to predict what will happen before the next election. It may be that that policy is settled – we just have to wait and see.”He said that he was also backing a further relaxation of leadership rules to make it even easier for leftwing candidates to run. While the last Labour conference changed the rules to ensure candidates only needed the support of 10% of Labour MPs rather than 15%, Lansman said the change “doesn’t go far enough”.

He also said he would back a move to ensure MPs needed the support of 66% of their local Labour and trade union branches to be automatically reselected as an MP, rather than a simple majority under current rules.

“I don’t think that any hardworking MP has anything to fear,” he said. “Having greater say in election is not about deselection, it is about greater accountability.”