The party holds just 18 seats in Britain's 650-seat parliament but has cast itself as the only "Stop Brexit" party, hoping to pick up votes from the 16 million who voted to remain in the European Union in 2016 and win enough seats to form what would be an unprecedented Liberal Democrat government. "Yes, we will revoke Article 50".
The clear stance on Brexit was cemented when members at the Liberal Democrat conference in Bournemouth voted overwhelmingly to support a motion to revoke Article 50 it the party gains a majority in a general election.
Sam Gyimah, a former Tory minister, became the sixth MP to switch allegiance to the party this year and some polling companies predict the Lib Dems could take as much as a fifth of the vote at the next election - up from just 7% in 2017.
"I recognise not everyone agrees with the Lib Dems on this".
He told delegates: "The Tory right who've taken over that party, they like nothing more than to bang on about incarcerating more and more people who break the law but strangely they think there should be a different approach to their law breaking".
Previously, the party backed a "People's Vote".
The Liberal Democrats remain a third-party group in what has been hitherto traditionally a two-party political system, making it highly unlikely there being a Lib Dem government as a result of a snap election.
He said: "I would hope that we get more than 40 seats at a general election. but we know from the internal polling that if we move from the position that we're in and say there is a 1.5% to 2% swing we can get up to 100 seats and if there's a 5% swing towards the Liberal Democrats through the course of the campaign 200 seats are in contention - but who knows what will happen?"
Mr Umunna said: '(Clement) Attlee and (Ernest) Bevin helped found Nato, Jeremy Corbyn and those around him want to abolish it'.
The Lib Dems have seen their numbers build in Westminster, with several former members of British Prime Minister Boris Johnson's Conservative Party joining the ranks.
But he said Saturday: "Boris Johnson created a stark choice for moderate, progressive MPs in the Conservative party - to accept a no deal Brexit or walk away from public life".