Nick Clegg and his Liberal Democrats have gathered for their spring conference in York under the imaginary banner: "We're not the Tories or Labour - and we're definitely not Ukip".

The third party has always had the problem of defining itself against the big two. Now it finds it is the fourth party and is desperately trying to differentiate itself from the big three.

So it is no surprise that this weekend's conference has seen business secretary Vince Cable laying into David Cameron's promise to hold an "In-Out" referendum on Europe, calling it "irresponsible" and saying it blights foreign investment.

Nor is it shocking that deputy prime minister Nick Clegg has focused on attacking Nigel Farage and says he's looking forward to their head-to-head TV debate ahead of the European parliament elections in May.

Cable kicked off the rally saying: "Nick Clegg is right to take the fight to UKIP. But as much damage is also being caused by the Tories in their Ukip-induced funk.

"The possibility of a Tory government after the election and the uncertainty around a referendum in three years' time is creating a blight over the business investment which we desperately need to support recovery and jobs."

He said businesses were telling him, on a daily basis, that they would pull out of the UK if it leaves the EU, naming Vauxhall, BMW Mini, Ford and Nissan, the aerospace industry and the City with British, Japanese, American, Indian and German firms all voicing fears.

A many as 3.5million jobs are at risk, he said, adding: "It is seriously irresponsible to put these jobs at risk."

Meanwhile, Clegg will make two platform appearances at the conference which he will use to further differentiate the party from the Tory half of the government, specifically by insisting he is the only political leader fully committed to the EU.

The conference delegates will hear plenty of talk about how the economic recovery would not be happening without the Lib Dems in government and claims the Tories are planning to once again steal credit for the party's policy of increasing the lower tax threshold.

The rally comes against a bleak background for the party as polls and by-elections show they are running fourth and there is the real possibility they will be humiliated in May's European elections, thus the focus on the issue in York.

Yet delegates are still in an upbeat mood. The majority have come to terms with the realities of coalition and the fact that they have finally become a grown up party of government, something they could only dream of for decades.

But there remains a dark cloud on the horizon. While party bosses and members may well believe there could be another hung parliament like in 2010, the chances they will be asked to form a second coalition appear to be diminishing rapidly.

No one really believes Ed Miliband or David Cameron will rule out a coalition before the election but, equally, there is a growing sense in both the big parties that they might prefer to try to govern without Clegg's troops, even if that means holding a second general election in, say, the autumn of 2015.