Campaigners concerned about Theresa May’s Brexit plans may not have pockets as deep as Conservative donors, but they are also drawing up plans to ensure there is a cohort of MPs in parliament prepared to oppose a “hard Brexit”.

Former shadow business secretary Clive Lewis, former Lib Dem leader Nick Clegg and Green party co-leader Caroline Lucas are among 16 candidates who will benefit from support from the Best for Britain group, Gina Miller’s campaign to encourage tactical voting and guard against an “extreme Brexit”.

Miller’s campaign, which has raised almost £400,000 to assist its tactical voting drive, is also planning to fund measure aimed at boosting turnout among young people and women.

Direct cash donations will be made to some, but not all, of the chosen candidates. The cross-party campaign will carry out its own activities in the constituencies of its “Best for Britain champions”, supporting them with local ads, endorsements, visits, Facebook ads and digital marketing.

There are risks for Labour candidates who accept the endorsement. The party’s official line is that Brexit will happen, but the group only backs those willing to keep “the options open for the British people”.

The candidates will also have to pass several tests. They must support “a real choice on the final deal and ready to fight Extreme Brexit”; they must be fighting a winnable seat that is not a dead certainty; and they must have an “immaculate track record” in public office.

The list of candidates may grow or shrink as the election approaches. The campaign has taken advice from the founder of tactical voting initiative that helped propel Justin Trudeau’s Liberal party to power in Canada.

Best for Britain will be touring some marginal constituencies and running local advertising campaigns in key seats in an attempt to limit the Conservative majority after the election. Some predictions put Theresa May on course for a three-figure majority.

The Labour candidates being backed are Kevin Brennan in Cardiff West, Fabian Hamilton in Leeds North East, Rupa Huq in Ealing Central and Acton, Peter Kyle in Hove, Clive Lewis in Norwich South, Kerry McCarthy in Bristol East, Tulip Siddiq in Hampstead and Kilburn, Andy Slaughter in Hammersmith, Jo Stevens in Cardiff Central, and Gareth Thomas in Harrow West.

The five Lib Dems are Tom Brake in Carshalton and Wallington, Mark Hunter in Cheadle, Sarah Olney in Richmond Park, Lisa Smart in Hazel Grove, and Clegg in Sheffield Hallam. One Green candidate – Lucas, Brighton Pavilion – is being backed so far.

Miller, who successfully challenged the government over giving parliament a say on triggering the Brexit process, said the group had been very selective in its choice of candidates.

“We are going to cause some waves and some angst with some people who think that because we are friends, we will be picking them, but we are going to be quite stringent on what we do,” she said. “Candidates must be backing that real choice on the final, meaningful vote at the end of the deal. That is what they will be very vocal about.

“They are candidates who will fight for a real choice on the final deal, who are fighting a winnable seat but not a dead certainty, have an immaculate track record in terms of personal conduct and principles and, finally, are opposed to an extreme Brexit.

“This list is the first of a possible three-phase list. In the last 24 or 36 hours, some of those names will switch when we come to recommend votes. We saw in the French election that some people actually left it until walking into the polling station to decide. It won’t be a fixed list; it will be fluid.”

Miller said there was such a hostile atmosphere from some quarters to politicians raising concerns over Brexit that some candidates had concerns about being endorsed by the group.

“We have had some conversations with some people where they have said, ‘Personally, I would like to say yes, but I really need to go away and think about this – I need to think about the other candidates in the party, friends in other constituencies’,” she said.

“Being supported by an organisation headed by me will have its own media impact, and that has to be in the equation – and the fact that they will be accused of ‘unpicking Brexit’, because there is a lot of misunderstanding about what we are trying to do.

“There is still this idea that it is about reversing Brexit. But we are very clear that this is not about going backwards; it is about going forwards.”

She said she had thought “long and hard” about being an election candidate herself, but decided it was not the right time. “I’m not saying never, but I can’t foresee [running for parliament] in the next few years.”