Video report by ITV News Political Editor Robert Peston
The party said scrapping Brexit will generate billions for public finances because they say remaining in the European Union will mean the economy will grow faster than under Brexit, and will lead to higher GDP.
Speaking at her campaign launch in central London, Ms Swinson launched attacks on Labour and the Conservatives, insisting she would make a better prime minister than either Jeremy Corbyn or Boris Johnson.
ITV News Political Editor Robert Peston explains how the election and potential coalitions will be 'desperately unstable'
Ms Swinson told supporters: “The Liberal Democrats are the only party standing up to stop Brexit and build a brighter future for the UK.
“Brexit has taken far longer and cost far more than anyone said it would.
"But any form of Brexit will damage our jobs, our economy and our public services, starving them of vital cash as the economy struggles along."
She added: “The Liberal Democrats will stop Brexit and then use the £50 billion Remain bonus to invest in our public services.
“Every vote for the Liberal Democrats is a vote to stop Brexit, build a brighter future and invest the £50 billion Remain bonus in our vital public services.”
The Lib Dems forecast the economy will be 1.9% larger in 2024-25 if Brexit is stopped than it would be under the Conservative Government’s Brexit deal. They said this means government receipts will be higher too, providing a £50 billion "Remain bonus" over the next five years.
The party said the £50 billion figure reflects the total extra current receipts over that period.
'I would be a better prime minister than Johnson or Corbyn'
Ms Swinson used the launch of the Liberal Democrats' campaign launch to position herself as a pro-Remain prime ministerial candidate - something she said which neither Labour or the Conservatives were offering.
"I am excited for the weeks ahead, I am excited to stand here as your candidate for prime minister, because a Liberal Democrat government will stop Brexit and build a brighter future," she said.
Ms Swinson insisted the upcoming December poll was not a straight choice between the "red and the blue team", as both Labour and the Conservatives wanted to deliver on Brexit.
She added: "I never thought I would stand here as a candidate for prime minister but when I look at Boris Johnson and Jeremy Corbyn I am absolutely certain I can do a better job than either of them."
Ms Swinson was particularly scathing of Mr Johnson during her speech, saying that he was not fit for Number 10.
"Boris Johnson only cares about Boris Johnson," she said. "He is not fit to be prime minister.
"He has always put his own interest ahead of the national interest. Even way back to when he was going to decide which side he was going to back in the referendum campaign.
"Most people picked a side on what they genuinely believed, and argued that position. Not Boris Johnson. He wrote two different columns and decided which one would give him the best chance of becoming prime minister.
"What more do you need to know that shows you he's unfit to be prime minister?"
Swinson also ruled out another Liberal Democrat coalition, saying she would not join forces with a Labour government offering another EU referendum.
She said: "I am absolutely categorically ruling out Lib Dem votes putting Jeremy Corbyn into Number 10.
"On so many grounds he is not fit to be prime minister."
The pledge comes after Ms Swinson said she will launch legal action against ITV if she is not included in a televised debate between Jeremy Corbyn and Boris Johnson.
The Lib Dem leader said debates about who will be the next prime minister “cannot and should not exclude the only woman leader who is able to be the next prime minister”.