Nick Clegg has claimed that the struggle on the right of British politics caused by Ukip's surge was pulling David Cameron away from the centre ground and making day-to-day progress in the coalition government more difficult.

Interviewed on the eve of the local elections, and facing the prospect of coming fourth in terms of share of the vote behind Ukip, the Liberal Democrat leader and deputy prime minister vowed to "dig in my heels and make sure the centre of gravity of the government as a whole does not get pulled rightwards due to the internal dynamics of the Conservative party".

Clegg cited Conservative policies on welfare, Europe and climate change as the three pre-eminent examples of Cameron being pulled right, and conceded that his coalition partner was no longer the same political animal as presented before the 2010 general election.

He admitted that Cameron's positioning "makes it more complex to make progress in areas where initially we thought it would easy to make progress", disclosing he has recently had to spend more time to secure agreements on the green agenda in government than on any other issue.

It is the first time that Clegg has acknowledged that the rightward shift of the Conservatives is making the functioning of government more difficult. But he also argued that there was a shift to the extremes of in British politics that was an inevitable consequence of the squeeze on living standards. "My job is to make sure that my party does not get pulled left or right as Cameron goes right and [Labour leader Ed] Miliband goes left," he added.

Clegg said that it was a rule in politics "as night follows day" that "when people are anxious and insecure about the future, and you have a prolonged periods of social anxiety, siren voices on left and right become very attractive. On the left it is attractive to say you don't need to make any difficult economic decisions. On the right it becomes tempting to blame it on everyone else – foreigners, immigrants, take your pick."

The polarisation of politics, Clegg added, was "an observable trend every time any country in the developed world goes through difficult times," adding that he did not think it was necessary to "respond by chasing. Not only do I think the centre can hold, it must hold. I think that is one of the vocations of the Liberal Democrats."

Expressing some pretty strong personal disillusionment with the dynamics of Tory politics, the deputy prime minister argued that "the Conservative emphasis before 2010 was to be centrist, compassionate, green party, but it is returning to some pretty traditional Conservative signature tunes that I totally understand they think are necessary to strengthen Conservative defences against Ukip. There is a real struggle on the right of British politics".

In a fresh sign of the difficulties the rise of Ukip is having on the functioning of the coalition, Clegg's aides indicated they would reject any attempt by Cameron to introduce legislation in this parliament to hold a referendum on Europe in the next parliament. A Clegg aide said: "We already have legislation setting out the circumstances in which we would hold a referendum. The idea that we need a new law now to set out the terms of a referendum in four years time suggests someone's priorities have gone astray".

The Conservatives fear they are set to lose as many as 500 seats in the elections, higher than the estimate offered by independent observers. But the Ukip surge is a wild card that makes predictions perilous. In recent weeks, Downing Street has made a series of rightwing announcements to stem the Ukip tide ranging from ending aid to South Africa, tightening jail conditions, cutting benefits for immigrants and threatening to leave the European convention on human rights on a temporary basis.

The Lib Dems are hoping that the political turbulence will decline once the elections recede, but there is a private fear that the Ukip surge will continue until the European elections next year at a minimum, so placing greater strains on the coalition.

Elsewhere in the interview, Clegg emphasised the extent to which he had pressed for a slowing in the pace of the government's deficit reduction strategy, pointing out cuts were now being implemented more slowly than in many other economies. He said: "As a proportion of GDP, our fiscal contraction plans this year and next year are less than those of Barack Obama, Mariano Rajoy and François Hollande."

He added: "I have personally never presented plan A like a tablet of stone. It is a series of judgments that we have applied flexibly and pragmatically. In one sense, it is resolute and unflinching in terms of our destination, but in another sense it is very flexible about how long we get there. If it takes longer, it takes longer. The idea that there is a choice between a cruel and unbending plan A and a cuddly and uncontroversial plan B is wholly false."

He also revealed there was some flexibility in the 2015-16 spending round on whether unprotected spending departments can offload some of their spending commitments on to protected departments. He said: "People in Whitehall, and those watching Whitehall, are operating according to total arcane Whitehall departmental silos. The world is not segmented according to Whitehall segmented boundaries.

"What I care about is making sure the big judgments on protecting the budgets of health, overseas development and schools is retained, but how that falls across departmental boundaries is secondary."

But he added that there would be no attempt to redefine the aid budget in such a way to mask cuts: "I can rule out that we are going to reinvent the definition of overseas development in a way that breaches international definitions".

Clegg vowed that Britain would not leave Afghanistan early despite the death of three British soldiers in Helmand province on Tuesday. "It is very important that just as we went in together so we should come out together," he said.

The deputy prime minister promised to be guided by public health professionals on minimum alcohol pricing and plain packaged cigarettes, two public health issues that may feature in the Queen's speech later this month. He said no final decision had been made to reject minimum alcohol pricing: "If the evidence can show the public health benefits outweighs the doubts, then I would side with the public health arguments, but they have to be compelling arguments."

Clegg urged Labour to rethink its economic approach after a rocky week that featured a poor performance by Miliband in a Radio 4 interview. "Personally I think Labour is walking into a trap," Clegg said. "They are being so hyperbolic and breathless in their accusations against the coalition government that they are giving Labour supporters the impression that it will be terribly easy to do something different. At some point, they are going to overcome their schoolboy posturing and say what they are going to do.

"When you have the second largest deficit after Greece and when you have slowed the deficit reduction timetable as we have, and when you have deficit reduction plans that are the same or less than some of your major European competitors, the idea that Labour seems to be peddling that there is an economically literate alternative available to us misses the point."