Nick Clegg will build up to his head-to-head European election debates with UK Independence Party (Ukip) leader Nigel Farage by promising today to take on "backwards-looking politics" and making the case for membership of the EU.
The Deputy Prime Minister will acknowledge the EU needs reform, but in a swipe at his Tory coalition colleagues will stress that the UK cannot demand changes with "one foot out of the door".
He will warn that "pulling up the drawbridge" would wreck the economy, make it harder to catch international criminals and hamper efforts to tackle climate change.
Mr Clegg wants to convince the electorate that May's European poll boils down to a choice between Ukip - who would take the UK out of the EU - and the Liberal Democrats, who are unashamedly in favour of retaining links with Brussels.
But his gamble on Europe, with the Lib Dems trailing fourth in opinion polls, was criticised at Mr Clegg's own party conference, with one activist telling him "no one on the doorstep cares".
Mr Clegg will say: "Forget the lazy assumption that, in the court of public opinion, the Eurosceptics will automatically win. There is nothing automatic about election results."
He will point to a Lib Dem Twitter campaign calling for people to express their support for remaining in Europe, saying it showed people wanted "hope" about the country's future.
"They don't want bile. They want jobs. They want our country to have influence. They want opportunities. Ultimately, they want hope."
Mr Clegg will take on Mr Farage in two debates ahead of the contest, and in his speech to the Lib Dem spring conference in York, Mr Clegg will acknowledge the reasons behind the rise in Ukip's fortunes, which have seen them push his party into fourth place in opinion polls.
The Lib Dem leader will say the "shattering collapse of the basic assumptions by which successive governments have run our economy" on top of a "crisis of confidence" about the West's future caused by the rise of China and other emerging nations had led to an "entirely understandable but dangerous urge to turn inwards".
He will say: "If anyone doesn't believe it, just glance across the Channel at our European neighbours, where a number of extremist parties are on the rise.
"We are lucky. Thankfully we do not have the same extremism here in the UK. But that's not to say the fight isn't on for the future of our country too."
Although he will not mention Ukip or Mr Farage by name in his speech, he will warn: "An ungenerous, backwards-looking politics has emerged in Britain. The politics of blame has found an acceptable face: it wears a big smile and looks like someone you could have a pint with down the pub.
"So I'm drawing a line in the sand. I am going to defend the tolerant and modern Britain we love, and I am going to start by showing people what's at stake at the upcoming European elections: do you want Britain in Europe, or out?
"That's the real question in May. One party wants out. Another is flirting with exit. The other lot don't have the courage of their convictions on this - they're saying nothing at all.
"The Liberal Democrats are now Britain's only party of In. The only party out there explaining the clear benefits of Britain's place in Europe. The only party giving people the facts."
Setting out why the Lib Dems are for remaining in the EU, he will say: "In, because Europe is our biggest export market and vital to British jobs. Because pulling up the drawbridge is the surest way to wreck our economic recovery.
"In, because in the fight against climate change, and in a world where some of the biggest players are also the biggest polluters, Europe's nations can only make a difference if we work together.
"In, because co-operation between our police forces is essential for catching the criminals who cross our borders. Crime crosses borders, so must we.
"In, because Britain stands tallest in the world when we stand tall in Brussels, Paris and Berlin."
Prime Minister David Cameron has set out plans to renegotiate the UK's links with Brussels and hold an in/out referendum on EU membership by the end of 2017 if he is returned to No 10 after the general election.
Mr Clegg will acknowledge the case for change "isn't about some starry-eyed affection for the EU - of course it needs reform".
"But you can't change it with one foot out the door. You change it by taking your place at the table - which is where you protect Britain's national interest too."
Answering questions from activists yesterday, Mr Clegg was questioned by councillor Jill Hope about the conference slogan "In Europe, in work", claiming that the issue was of little interest to voters.
"No one on the doorstep cares," she said in the question she submitted to the Deputy Prime Minister.
Before Mr Clegg closes the event, the spring conference will also debate proposals for a "digital Bill of rights" in response to US whistleblower Edward Snowden's revelations about state surveillance.
Party president Tim Farron will say: "The intelligence services can now call on powers which would have been completely unimaginable even 10 years ago.
"But our current legislation - the Regulation of Investigatory Powers Act, the Intelligence Services Act and the Telecommunication Act is all more than a decade old...Our laws in this area are quite simply not fit for purpose."
"Blanket surveillance of any kind is not the answer to combating terrorism. Our country has well established rules on the deployment and use of communications interception.
"I do not value someone's electronic conversations as any less private than their letters or phone calls. We should revise and extend our current legal safeguards to electronic communications as well.
"Freedom is the very cornerstone of our party. We exist to safeguard a free, fair and open society. But that society has been challenged by the state's imposition of blanket surveillance on us all.
"We must begin the process of restoring our liberty and ensuring that Britain is the country we can feel proud of again."
The motion would ensure that the Government does not undertake the bulk collection of data and only accesses the metadata - details of who, where and when contact took place - or content of communications if there is suspicion of unlawful activity.
But it also criticised Mr Cameron's efforts to protect children from online porn, warning that "Government-supported filtering of the internet will prevent people from accessing legitimate information and educational resources whilst giving parents a false sense of security".