Momentum-backed Labour candidates will dominate England and Wales’ tightest marginal seat battles even though the leftwing group has won only around a third of selections overall.

The leftwing grassroots group has recorded a string of victories in selection battles where Labour has the greatest chance of winning seats at the next general election, including those of the home secretary, Amber Rudd, (Hastings and Rye), the former minister Anna Soubry (Broxtowe) and the Conservative vice-chair Ben Bradley (Mansfield), which Labour lost at the last election for the first time since 1922.

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The group scored another victory last week in the Newham mayoral race, where it backed Rokhsana Fiaz over the long-time incumbent Sir Robin Wales, though Fiaz had widespread support across the party, including from trade unions and the centre-left group Progress.

However, across almost 50 seats that have been selected so far, the picture is more mixed, according to a Guardian analysis. Around 30% of the total selections have been won by candidates either officially promoted by Momentum or with close links to the group, though it has only officially contested 33 selections.

The majority of those Momentum victories have been in the 20 most marginal seats – those with majorities of less than 2,000 – including Stoke on Trent South and North East Derbyshire, which were lost by Labour in 2017.

The group’s backing has sometimes been opportunistic. In Calder Valley, Momentum backed the councillor Josh Fenton-Glynn, who had been critical of Jeremy Corbyn in 2016. Joe Bradley, the group’s organiser in the north-east, said it was not driven by ideological purity.

“He [Fenton-Glynn] is not someone who you would naturally expect us to support, but it’s a seat we can definitely win and we looked at who was the best candidate,” he said. “You have to be pragmatic. Ultimately, our goal is a Jeremy Corbyn Labour government. He’s the best person to win the seat; for us that’s all that matters.”

Over the coming months, Momentum’s campaigning is set to focus on seats with high-profile MPs the group believes it has a chance to defeat, rather than just seeking out typical marginals.

Conservatives being targeted include Greening, Rudd and Soubry, as well as those with far bigger majorities, including the former Tory chair Grant Shapps and the foreign secretary, Boris Johnson. The group even plans a day targeting Jacob Rees-Mogg, who has a majority of more than 10,000 in North East Somerset.

The analysis shows 60% of the candidates selected so far to fight Labour target seats have been women, including the former MP Julie Hilling in Bolton West and Sadiq Khan’s former IT adviser Charlotte Holloway in Plymouth Moor View.

Ensuring women fought more marginal seats was a key aim of the party’s national executive committee, which has enforced all-women shortlists in 46 of Labour’s top 76 target seats.

Campaigners have previously raised concerns about the lack of black and ethnic minority candidates and at least two BAME candidates from 2017 were prevented from running again because of the all-women shortlists. Just two ethnic minority candidates have been selected so far, both of them men.

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Stoke on Trent South’s Mark McDonald, a lawyer and close ally of the Labour leader, is the only BAME candidate among those who will fight a seat with a majority of less than 1,000. The Lancashire Labour council leader, Azhar Ali, will fight the Tories in Pendle.

Among the former BAME candidates in marginal seats who will have to stand aside are Bilal Mahmood, who slashed Iain Duncan Smith’s majority in Chingford and Woodford Green, and councillor Kuldip Sahota, who fought the 2017 election in Telford and was replaced at the weekend by Katrina Gilman, an LGBT campaigner and union official.

Putney will also have an all-women shortlist, ousting the 2017 candidate Neeraj Patil, an A&E consultant who slashed Justine Greening’s majority from more than 10,000 to just over 1,000.

The Labour MP Tulip Siddiq said the numbers were unacceptable. “A key reason why I joined the Labour party was seeing MPs such as Diane Abbott smashing the glass ceiling of representation for BME women,” she said. “Nobody can credibly claim there is a dearth of exceptional BME female voices out there. It is imperative that Labour gives them a chance to flourish.

“If we truly want to continue being the party of equality, we need to take intersectionality into account and ensure we support candidates who are most marginalised in society.”

Labour said the party’s forthcoming democracy review would examine new ways of promoting diverse political representation in Parliament. “We are proud that our party has more BME MPs than all other political parties combined,” a spokeswoman said.

The review will come as Labour’s national executive committee prepares to appoint Jennie Formby as the party’s new general secretary. Sources said the organiser for the Unite union –who is the choice of Jeremy Corbyn – will comfortably beat the former teaching union leader, Christine Blower. The decision will come from a secret ballot after the pair give presentations to the group.

The move triggered a series of resignations within Labour headquarters, including two key figures in the general secretary’s office, Julie Lawrence and Tracey Allen, head of the compliance unit, John Stolliday, director of policy and research, Simon Jackson and other key figures, Dan Simpson and Neil Fleming. They follow the departures of the former general secretary, Iain McNicol, and Emilie Oldknow, who many had tipped as a possible successor.

Some claimed people were leaving because they did not want to work with Formby but others insisted the changes were amicable and normal given a swap-over at the top. They pointed out that they were six out of 420 people employed by Labour.

• This article was amended on 20 March 2018 to correct the year Labour lost Mansfield from 1885 to 1922.

