Business pressure on the Government represents the best chance of softening Theresa May's approach to Brexit, Tim Farron said.
The Liberal Democrat leader launched an appeal to corporate Britain, urging business leaders to drop the Tories and switch support to his party in an effort to protect single market membership.
Accusing the Prime Minister of "twisting" the will of the British people by backing a hard Brexit, Mr Farron told business donors: "If you are giving the Tories so much as a penny you are funding your own funeral."
The Lib Dems received more in donations than Labour in the final three months of 2016, and Mr Farron is now aiming at the Tories, arguing that supporters should switch now, before Brexit talks get under way.
He told the Press Association: "Everybody in business knows that leaving the single market will be massively damaging to our economy, to individual firms, to families and to the Chancellor's own revenues.
"So what can we do to prevent a hard Brexit, to keep us in the single market?
"You need to persuade Theresa May and Philip Hammond they need to listen to business voices rather than to those extreme pro-hard Brexit voices that are currently within the Tory party.
"How would you do that? If you are in business, you should drop the Tory party today like a hot brick.
"If you are giving the Tories so much as a penny you are funding your own funeral.
"Don't spend a week or two thinking about it, you need to do it right now.
"It is literally the only language they understand."
Switching that funding to the Lib Dems instead "would have double the impact", he suggested.
In an echo of New Labour's "prawn cocktail offensive" to win support from the corporate world, Mr Farron said: "We are talking to businesses, we have an entrepreneurs forum which is growing hugely, a business advisory group with some very strong people on it.
"Our outreach into the business community is significant."
Mrs May had taken business support for granted, he said, which was "offensive and wrong".
She was not "enacting the will of the British people but - if one is being generous to her - she is interpreting it or even twisting it".
Mr Farron added: "It was not on the ballot paper and not inevitable that we leave the single market.
"So it is perfectly democratic and perfectly right for all of us to use what power and influence we might have to make sure that she doesn't make that choice."
The Lib Dems gather in York for their spring conference on Friday and senior figures believe the Brexit referendum has pressed the "reset button" for the party with the electorate, with coalition-era troubles and tuition fees no longer raised on the doorstep.
Mr Farron said the Lib Dem membership had doubled since the general election, reaching 82,500.
"It feels like the wind is in our sails," he said, but with his own party having just nine MPs and Labour struggling under Jeremy Corbyn, the Tories could be in power for a generation.
"At the moment, the Labour Party are not only a poor opposition to them, they are in no way capable of forming an alternative government either," he said.
"So it's either us or you are left with the Tories for as long as you can see into the future.
"I think that is utterly unacceptable so our recovery for the sake of the country needs to be big and it needs to be quick."