The Liberal Democrats are fighting to hang on to their seats this general election but Clegg is proud of his achievements in the coalition

It seems a bit cruel to torment him with the memory, but we remind him anyway. Around this time five years ago, “Cleggmania” was all the rage. The polls had the Lib Dems surging at over 30% and one Sunday newspaper even ran a hyperbolic headline describing the Lib Dem leader as more popular than Winston Churchill. “I know. It seems it’s happening all over again,” he smiles, taking the tease in good part. “No, not.”

Clegg’s personal ratings have picked up a bit since the campaign started, but they remain deeply negative. The polls put the Lib Dems at about a third of the support that they won in 2010. He readily acknowledges that he is not on a “swashbuckling Don Quixote ride” to try to paint Britain yellow. “We’re not going to cut through in a lot of those parts of the country where we are not present in people’s lives.”

For the Lib Dems this is “a defensive election”. They are fighting a tightly targeted battle for survival, with their ambitions limited to trying to hang on in places they already hold. In constituencies that have Lib Dem MPs, claims Clegg, they do “get a very, very good hearing. I’ll tell you why. A very good, old-fashioned reason. We’ve got an incredibly good story to tell, stuff that really resonates with people.”

He seeks credit both for the Lib Dem policies that they have managed to implement in government and for keeping a leash on the Tory rottweiler. “We’ve stopped them time and time and time again from doing highly regressive things.”

One of the more refreshing dimensions of Clegg is that he is readier than most politicians to admit to mistakes. He will defend to his last breath the decision to go into coalition with the Tories, but he does concede that he was poor at communicating the consequences. “I think it is a forgivable error, but it was nonetheless an error. I don’t think we invested nearly enough time explaining what the plot was. You can’t expect the British people to accept controversial, difficult, downright unpopular decisions unless you have laid the groundwork.

“We took huge decisions at a gallop and we didn’t explain enough why. We should have spent a lot more time explaining why.”

Some colleagues – Vince Cable for one – think Clegg made another mistake right at the beginning when he stood in the Number 10 Rose Garden joshing with David Cameron as if it was a hot bromance rather than a marriage of necessity. It was too lovey-dovey with the Tories and the Lib Dems were punished for it by voters who felt betrayed. This Clegg passionately disputes.

“I couldn’t disagree more. People have very short memories. They forget the hysterical predictions about what coalition would mean. ‘Locusts are going to descend from the skies. The sun is going to be blotted out. A coalition government would be an absolute disaster for this country.’

“It was essential in the early stages of this government that we did one simple primary-colour thing which was to show that the government could function. You had to show that it was a government who could govern in the national interest and for the full duration of the parliament. You had to show that. And, by the way, either myself if I’m in coalition again, or my successors, won’t have to do that next time because we’ve shown that coalition works. We’ve proved it.” The price for the transition from party of perennial protest to party of power has been very high. Even the most optimistic Lib Dems expect their contingent of MPs to be dramatically reduced. Some polls have even suggested that Clegg himself is in serious jeopardy of losing his Sheffield seat. As we take the train for a day’s campaigning in the city, he expresses confidence that he will hang on, but acknowledges that it is a tough fight which is consuming a lot of his time.

Yet even a diminished band of Lib Dems could be pivotal if the polls are right and the next parliament is hung. The Scottish Nationalists are pledged to do anything in their power to keep out the Tories. Labour says it won’t deal with Ukip. That might put the Lib Dems in the pivotal position as the smaller party which could deal with either the Tories or Labour. Clegg might again be the kingmaker.

He denies that his secret preference is for another coalition with the Tories. “Just ludicrous,” he says. While he has some harsh words to say about the “fiscal illiteracy” of Labour, his most fierce language is directed at his current partners in government.

He is most animated by the Tory plan to find £12bn from unspecified cuts to the welfare budget. He calls the chancellor, George Osborne, “very dangerous” and accuses him and Cameron of peddling “a dangerous deceit” – the most lacerating description of the prime minister and chancellor that has ever come out of the mouth of the Lib Dem leader in the past five years.

“It is impossible to do that without hitting children, without hitting the poor, without hitting the disabled. It’s wilfully regressive.”

Five years of growing familiarity with the Tories have bred some cold contempt. “I’ve no doubt they’re doing this because they’ve got some focus groups which say that welfare cuts are popular. What they misunderstand is that people distrust their motives – and for very good reasons, because if left to their own devices their motives are always to get the poor to pick up the tab. This is why – despite rolling in cash, despite slavish support from the press – they’re still not going to win. Because people don’t trust them to act fairly. A lot of decent, moderate Tories do not like the idea of beating up on the poor.”

It is a message tailored to try to appeal to centrist voters in those seats where the coalition partners are fighting each other, but Clegg speaks with an animation that suggests he also really means it. He says Tory welfare cuts would be “the big sticking point” with them in any post-election negotiations with Osborne and Cameron. He will “never accept” their plan. “I have battled day in and day out to make sure this government governs from the centre ground, through unbelievably difficult circumstances. I’ve taken hit after hit after hit. I would never accept a radical departure, a lurch to the right.”

Clegg is less emphatic about whether the Lib Dems would try to use a position of influence in a hung parliament to block the Tories’ promised referendum on membership of the EU. He ridicules the Conservatives for their constant shifts in position on Europe. “I have been in government with these people for five years and the way in which they have changed their tune on Europe every few months is impossible to follow.”

What he doesn’t say is that he would seek to veto their referendum pledge in all circumstances. “I’m not going to enter into red-line language.” This will add to the impression that the Lib Dem leader might concede a referendum to the Tories in return for getting things his party wants.

Does he like Cameron? “We can work together,” he replies. Would you go on holiday with him? Clegg reacts with incredulity. “With a politician? On holiday? God, are you kidding? I never go on holiday with politicians!”

We turn to Ed Miliband. Clegg declines to echo the Tory attack that the Labour leader is unfit to be prime minister. “He’s getting the full hairdryer treatment from the swivel-eyed brigade on the right. Been there myself.” He could work with Miliband? “Yeah, course I would. Course I would.”

His main sticking point with Labour is what Clegg calls “their lack of credibility” on deficit reduction, but he more generally accepts that there is a lot of policy overlap between the two parties.

Some Lib Dems feel that, after the pummelling they have taken, they shouldn’t be thinking about another coalition; they ought to sit out government after the election and recuperate in opposition. “Sort of lick our wounds,” he says. “I totally understand that. It would be irrational not to be wary of what it might mean for the Liberal Democrats if we were to go into coalition again given what’s happened.

“Of course, you should be wary. I’m very wary. But a fatwa on never entering government again would be quite contrary to what liberals believe. My own view is next time would be immeasurably easier for us than last time.”

Why so? “We’ve learnt a lot. I’m a lot tougher, a lot wiser. And because the country is getting used to it.”

After the pulverising that he and his party have taken since 2010, would he really want another five years as deputy prime minister? “Yeah. Yeah.”

Really? “If people were up for it.”

The train is pulling into Sheffield. Clegg prepares to campaign among the people who are most personally vital to him, the people of his own seat who will decide whether or not he is even a member of the next parliament.