Nick Clegg has challenged Ukip leader Nigel Farage to a public head-to-head debate over whether Britain should remain in the European Union.
The Deputy Prime Minister said he was "very happy and very keen" directly to pit the Liberal Democrats' strong support of UK membership against Ukip's desire to withdraw.
He threw down the gauntlet as the Lib Dems brace for what the party president predicts will be "the fight of our lives" in elections to the European Parliament on May 22.
Tim Farron has warned that the party's "very presence in the European Parliament could be at stake" as it contends with poor opinion poll ratings that consistently put the junior coalition party behind Ukip.
"I'll tell you what I'll do. I will challenge Nigel Farage to a public, open debate about whether we should be in or out of the European Union," Mr Clegg declared during his weekly LBC 97.3 radio phone-in.
"He is the leader of the party of out, I am the leader of the party of in," he said.
"And I think It's now time we have a proper public debate so that the public can listen to the two sides of the argument and judge for themselves."
"I hope he will take up my challenge to debate, once and for all, publicly, should we be in the European Union which I believe means that we have more people in work than would otherwise be the case, we keep ourselves safer because we can go after cross-border crime and terrorism, we can look after the environment in a way we can't on our own or do we do what Ukip wants, which is to pull ourselves out of the European Union and so jeopardise millions of jobs in this country?
"That is the choice. Let's have the debate in the open. And I am very happy and very keen to debate that with Nigel Farage directly."
Following Mr Clegg's challenge, a party spokesman added that " the terms and arrangements for any debate would have to be agreed by both sides".
There was no immediate acceptance of the challenge from Mr Farage - which Ukip said he would give in "full" when he appears on the same radio station tomorrow.
"Mr Farage would like to thank Mr Clegg for his kind invitation to a debate on the great issue of Britain's membership of the European Union," a spokesman said.
"Perhaps he could also let us know whether he has invited David Cameron and Ed Miliband too in order that the British people can see all their main political leaders argue their positions.
"If this challenge means that Mr Clegg is going to restore his backing for an In/Out referendum, which he gave before the last election but then withdrew afterwards, then it could be a significant moment in British politics."
Mr Clegg has already stated that Mr Farage should be allowed to take part in televised debates between party leaders during the 2015 general election campaign.
The Prime Minister has made clear however that he does not believe Ukip should be represented if agreement is reached to repeat 2010's groundbreaking TV showdowns.
The Times reported the Mr Farron had issued a stark warning about the threat faced by the Lib Dems in May as part of a submission to the party's spring conference next month.
" It's not just our party's place in Europe that is at stake, it's Britain's too," he wrote.
"We know Europe needs reform, and the best place to do that is from within. Only the Liberal Democrats are brave enough to make this argument and put British jobs and investment first."