Transport secretary Patrick McLoughlin is pressuring Labour to reaffirm its support for HS2, as the government shifts its case for HS2 away from quicker journey times.
Labour support for the £43 billion project is now judged critical to its success by coalition ministers, after David Cameron made clear at last week's Europe summit that it could not go ahead without opposition support.
As the government unveiled its fifth attempt at a cost-benefit analysis justifying the huge spending decision today, McLoughlin confronted Labour scepticism head-on in a speech in Manchester.
"Labour leaders in our great cities who know this project is right. They know that any threat to the new line is also a threat to the future of the north and the Midlands," he said.
"So let me say something very direct to those in the opposition who have learnt nothing from the past.
"You can't say one day you back better infrastructure only the next threaten to stop it being built. You can't go on claiming to want 'one nation' if you won't back the things that will bring it together. You can't play politics with our prosperity."
For now the party's frontbench will continue its support for the project, even as signs grow the shadow Cabinet is losing confidence in the scheme. Shadow chancellor Ed Balls has said there is "no blank cheque" for HS2.
But Labour backbenchers face a difficult dilemma over HS2 when the issue is voted on in the Commons this Thursday. The party's northern constituencies are all hopeful the spending could boost their local economies, but nationally in Westminster the party could benefit from scepticism - and find more voter-friendly ways of spending the money.
Shadow transport secretary Mary Creagh said this lunchtime: "Labour has always supported HS2 because we must address the capacity problems that mean thousands of commuters face cramped, miserable journeys into cities like Birmingham, Manchester, Leeds and London.
"However, we cannot give a government that is mismanaging this, or any project, a blank cheque. Our message to David Cameron is clear. Get a grip on this project, get control of the budget and get it back on track."
Meanwhile Conservative backbenchers informally led by former Welsh secretary Cheryl Gillan will vote against the government. Around 30 are currently expected to rebel later this week.
The debate will focus on the government's renewed cost-benefit analysis, which warned rail passengers face 14 years of weekend delays because of upgrading work needed to the existing line if HS2 is scrapped.
The report is failing to reassure the fears of those who think the cost of the project will exceed its current £43 billion price tag, however.
"As we expected, the government have pulled some random figures out of the air in a desperate attempt to con the public," Stop HS2 campaign manager Joe Rukin commented.
"As if by magic, they expect us to believe that after three years that the economic case for HS2 has risen like a phoenix from the flames. They surely must realise that everyone is going to see through this cynical attempt at spin."
Stop HS2 and others believe the final bill could total as much as £80 billion, but chief secretary to the Treasury Danny Alexander made clear at the weekend the costs would not increase again.
Speaking this morning, McLoughlin would only say HS2 without Labour backing "becomes a more difficult project".
He signalled a shift in the government's rhetoric on the benefits of the proposed Y-shaped network, which will fork at Birmingham before reaching Manchester and Leeds.
"We're saying why this addresses the issue of not just speed as far as travel is concerned, but also capacity," McLoughlin told the Today programme.
"The study we're publishing today shows how it has a benefit across the country - not just a benefit for the cities who get it.
"As transport secretary, all my colleagues come to me and say we want more services... what we need is a great boost in capacity and faster interconnections between our cities. That's what HS2 does."
HS2 will cost the taxpayer £43 billion and result in drastically reduced journey times up the spine of England. London to Birmingham will take 49 minute, down from 84 minutes, while London to Manchester will take just 68 minutes - down from 128 minutes.
Shadow transport secretary Mary Creagh, whose own constituency of Wakefield would feature on the route, adopted a guarded approach before the cost-benefit analysis publication.
"We must address the capacity problems that mean thousands of commuters face cramped, miserable journeys into cities like Birmingham, Manchester, Leeds and London but there can be no blank cheque and ministers must get a grip on costs," she said.