This Easter we find ourselves in the middle of a European election campaign in which the question at the heart of the European debate is finally being addressed: should Britain remain a member of the EU, or is it time to leave?

The isolationists have been allowed to peddle their myths unchallenged for decades. Of all Nigel Farage's far-fetched claims – and there are many – the most outlandish is the idea that Ukip's call for an exit is the insurgents' battle cry. European withdrawal is presented as a great revolutionary promise, held in stark contrast to the status quo upheld by a homogeneous political elite.

What poppycock. For a start, Farage is every bit the professional politician he enthusiastically reviles. He and I were elected to the European parliament on the same day in 1999. I left after five years. The Ukip leader is still there. More important, there is nothing remotely new about his party's ambitions. Ukip is simply the fresh face of a long-standing Eurosceptic establishment, supported by many in the Tory party and significant parts of the press.

The sceptics' free run over this debate must now be challenged. On 22 May every gain they make will propel Britain closer to the door. We risk finding ourselves drifting further and further away from our biggest export market, jeopardising our still fragile economic recovery and threatening millions of jobs. At a time when the world's economic powerhouses are reaching beyond their own borders and working more closely with their neighbours, we will be turning away from ours.

If we end up stumbling out of the EU our police will be denied the cross-border co-operation they enjoy at present – in effect asking them to keep us safe with one hand tied behind their back. At every international summit and on each of the great dilemmas thrown up by globalisation, climate change, trade, global finance, terrorism and organised crime, Britain will be increasingly isolated, its influence diminished in the world.

With such a bleak future now plausible, my party has decided to take on the Eurosceptic establishment directly. The Liberal Democrats are staunch internationalists. We understand that in order to thrive in the 21st century, our nation must continue to work with others. We are pro-European because we are pro-British: we are determined to protect Britain's traditions of engagement and leadership on the world stage. We are, as you will hear us repeat endlessly over the coming weeks, the nation's party of in.

For these reasons I challenged Farage to two live TV debates: he leads the party of out. Some have questioned the wisdom of my decision. The snap polls put me behind my opponent, and as a result I have been asked if I regret instigating these debates. The answer is absolutely not. I would meet the Ukip leader in front of the cameras every day between now and polling day if I could.

I understand the urge to simplify events like these into clear winners and losers: straightforward victory and defeat. This is politics at its most gladiatorial, after all. But prior to these debates the case for in was largely absent from the public debate. Twenty years of deliberate inaccuracies and entrenched mistruths are not going to be reversed in two hours. At the end of the day, I don't believe it's possible to win an argument unless you are prepared to have it in the first place.

The arguments for in must, of course, be made relevant, and we must also set out a compelling vision for a better EU. We can capture people's imaginations by asking them to choose the kind of country they want to live in: Great Britain or little England; a society that is proud of its cultural diversity and open to the world, or one that is intolerant, insular and closed. But equally we must speak to their legitimate concerns.

The EU is not perfect: we need more trade and less bureaucracy and red tape. Britain must use its influence to get the EU back to what it's good at – enabling nations to collaborate on the areas where we each achieve more by working together. Priority number one must be completion of the single market in services and digital industries, boosting high-growth industries and our competitiveness as a whole.

We need to end any unnecessary meddling. For instance, let's make sure that small firms which don't export, such as hairdressers and newsagents, are exempt from EU regulation unless there's a cast-iron reason. And let's get rid of the ridiculous waste – £150m every year – that comes with sending Farage and all the other MEPs to Strasbourg every month.

By owning the reform agenda we will make the case for staying in even more compelling. It is not true that the sceptics have all the arguments on their side. People intuitively understand the need for safety in numbers in an uncertain world, and remaining open and outward-looking speaks to the values of millions of British people.

What we need is more volume. The Lib Dems have started this debate – but we cannot win it alone. We want to work with others to deliver the firepower needed to challenge the Eurosceptic establishment. If Labour is still a pro-European party, it needs to come off the fence. Tory modernisers must risk the wrath of their backbenchers and speak out. Anyone who agrees that we are better off in – whether that's businesses, the crime-fighting agencies, progressive voters or people who care about the environment – make yourselves heard. If you are holding back, ask yourself: what are you waiting for? The fight is on. The threat is real. It's time to pick a side.