The investment banker, 52, launched Best for Britain late last month along with figures from the business, arts and charity sector, and even gained the backing of businessman Sir Richard Branson.
The group has been urged to work closely with another Remain group, Open Britain, which was formed shortly after the Brexit referendum last June.
It is mainly backed by former political Remain campaigners although it has now emerged the two groups find it difficult to agree on strategy.
With Theresa May expected to secure a landslide victory for the Conservatives, some Remainers have taken a gloomy outlook on their chances of having an influence on public opinion sparking in-fighting.
One unnamed source told The Times: “It’s not going well. We are all fishing in the same pool. There is a need for a realignment after the election.”
Mrs Miller, who successfully brought a case against the Government over its authority to trigger Article 50, is now backing a tactical voting drive.
She has already launched a crowdfunding page calling on the public to give funds to “stop extreme Brexit” and has already raised just under £400,000.
The money is being used to fund 16 parliamentary candidates who back Remain hoping to get elected.
Those already having got the seal of approval from Mrs Miller include the former Liberal Democrat leader Nick Clegg, Sarah Olney, the Liberal Democrat hopeful for Richmond and the Green Party’s Caroline Lucas.
Best for Britain has also targeted voters in more than 30 constituencies and has its roots in party politics.
The cross-party group, fronted by Lord Mandelson, is dwindling after the likes of Tory former Business Minister Anna Soubry and former Conservative Education Secretary Nicky Morgan quit.
They withdrew their support after the group published a hit list of mostly Brexit-supporting Tories it wanted to unseat in the election.
Open Britain sees itself as the “obvious place” for Remainers as it holds the data for the Remain campaign.