Nick Clegg and I smile genially at each other across the table of a standard-class train carriage. He is on his way to his constituency in Sheffield to talk about manufacturing. Pale-faced, pale-eyed and so tired he appears taxidermied, he looks like he could do with a holiday, except he's just had one – skiing in Davos with his children as the Libyan crisis escalated (for which he was lambasted).

Nick Clegg is the Tim Henman of politics: a decent man for whom Cleggmania represented the peak of his career, his Henman Hill moment. Then he became the Deputy Prime Minister and, shortly after, an effigy.

The carefree, cloud-cuckoo days of opposition, when he had a platform and little criticism, are long gone. At last year's Liberal Democrat spring conference, a fresh-looking and ebullient Clegg had gesticulated and boomed: "We see the same old broken promises. No wonder people feel let down." A year on, he was less combative, more ambivalent. His many critics pointed to his own broken promises and let-down voters.

Clegg concedes that it has been a "very sharp transition". "Of course it has had a dramatic effect on how I'm perceived, the kind of dilemmas I have to face," he says. "I don't even pretend we can occupy the Lib Dem holier-than-thou, hands-entirely-clean-and-entirely-empty-type stance. No, we are getting our hands dirty, and inevitably and totally understandably we are being accused of being just like any other politicians."

His point – and it seems a fair one – is that the British public voted, no one party won and that coalition government, by definition, is a compromise. "A whole lot of things are happening that would just never in a month of Sundays have happened without the Lib Dems there," he says. The morning of our meeting, he claims to have "squeezed out of [George] Osborne" a promise of a green investment bank, not simply a fund. "We've done more on liberty and privacy," he adds, "in the past ten months than Labour did in the past 13 years."

All this has done little to dilute the vitriol of his opponents. John Prescott has likened him to Jedward, the risible and tuneless twins from The X Factor. Ed Miliband has called him "a tragic figure", one too toxic to share a platform with ahead of the referendum on the Alternative Vote. Clegg insists that none of this bothers him. "I see it exactly for what it is. [Ed] is a perfectly nice guy but he has a problem, which is that he's not in control of his own party, so he constantly has to keep his troops happy and he thinks that ranting and raving at me is the way to do it."

Since joining the government, and in particular since his U-turn on university tuition fees, Clegg has had dog mess posted through his door and been spat at in the street. It must upset him. "No, well look, I'm a human being, I'm not a punchbag – I've of course got feelings."

He pauses. "Actually, the curious thing is that the more you become a subject of admiration or loathing, the more you're examined under a microscope, the distance seems to open up between who you really are and the portrayals that people impose on you . . . I increasingly see these images of me, cardboard cut-outs that get ever more outlandish . . . One thing I've very quickly learned is that if you wake up every morning worrying about what's in the press, you would go completely and utterly potty."

After ten months in government, he has a guardedness that did not exist in the days when he told Piers Morgan he'd had roughly 30 lovers. These days he is tightly managed. I have already had a pre-interview briefing with one adviser, and now Clegg's version of Andy Coulson, who is sitting to his right, is busy taking written notes of our interview, as well as recording it. When Clegg gets sidetracked, he prompts him, head down, pen poised over notebook, deadpan: "You were talking about what you've achieved . . ."

Everyone seems painfully aware that my task as interviewer is to catch him out, to get him to say the wrong thing. Clegg's task, like all politicians, is to rattle off rhetoric, to be evasive and as uncontroversial as possible, and to fill up the tape with unquotable patter.

All of which makes interviewing him excruciating. He continues: "What we've achieved so far . . . I think just having a government with two parties in it is already such a big new thing. I know it has been born in a blaze of controversy because of the difficult economic decisions we've had to take . . . but if we're lucky, people will look back on it in 20 or 30 years' time as quite a normal thing in British politics that politicians can actually agree with each other from time to time.

“That in itself is quite big and radical. In the week or two leading up to the general election, every single newspaper was screaming from the headlines: 'A hung parliament will be a disaster, coalition politics will be a disaster. Nothing will get done.' And the extraordinary thing is that now we're being accused of almost exactly the reverse – of doing too much."

Of doing too much? Or of being too Tory? Clegg's dilemma is that, on the one hand, he is in danger of being seen as too close to David Cameron and the Conservatives, and losing credibility with his party and voters. On the other hand, he can't be too distant, because that would be damaging for the coalition and a gift for the opposition and the press, which is constantly looking for rifts.

Before the election, Clegg let it be known that he had turned down an invitation to dine with the Camerons at their home in Notting Hill. He wanted to maintain a distance. Perhaps wary of looking like he fits too easily into the port-swilling, waistcoat-wearing Bullingdon Club set, he is still keen to present Cameron as more working partner than friend.

“We don't regard each other as mates and actually I don't think it would be a particularly healthy thing if we tried to become personal mates," he says. "I don't think a coalition works unless you have a very careful balance between mutual respect and civility and also a certain hardness, as at the end of the day you are representing different views."

I've heard that they play tennis together. "No, no – well, er, I think we've played one game of tennis. Of course we meet from time to time but it's always basically to talk about what we're doing in government."

Who won?

“Ah no, that's a state secret," he jokes. (Cameron won.)

Earlier, at my pre-interview briefing, Clegg's adviser Richard Reeves, the former head of Demos, characterised being in the coalition as like being in a marriage – you both get to know instinctively which are the no-go areas.

Clegg concedes that there are "some areas where we flatly disagree" with the Tories, such as on Europe ("I think you can't make sense of this world unless you work together with other folk in the European neighbourhood") and taxation ("Our reflexes as Lib Dems are to try to give tax breaks to people on middle or lower incomes, whereas traditionally they are more interested in trickle-down economics"), but denies that there are "no-go areas". "Look, we're on completely opposite sides of the fence on the AV referendum."

He refuses to concede that signing the pledge to vote against an increase in university tuition fees before the election was a mistake. "That would be a cop-out. I did it. And I have a rather old-fashioned belief that you've got to stand by what you've done and take the consequences, good or bad." He insists that it was not one of his main manifesto priorities anyway. "I didn't even spend that much time campaigning on tuition fees."

Instead, he says, he spent "every single day and every single interview talking about the four things that were on the front page of the manifesto – namely the pupil premium, two and a half million quid for disadvantaged kids; changing the tax system, so you don't pay tax on your first £10,000; political reform; and sorting out the banks and rebalancing the economy."

That's all very well, but given that the Lib Dems are only ever likely to be in government as part of a coalition, how will he deal with pledges made in future election campaigns? Will there be pledges with caveats, depending on which party he clambers into bed with next? "I think that we need to be clearer about what are the really big, big priorities."

After his capitulation on tuition fees, there are many who now fear that nothing is sacred for the Lib Dems. He denies this. "If the Conservatives wanted to become as authoritarian as Blair and New Labour, I wouldn't have it – but it wouldn't happen, as it couldn't happen with us in [the coalition]."

Clegg is emphatic that he will not allow the Tories to disempower the Lib Dems' much-loved European Court of Human Rights. The problem with being in a coalition government is that it acts as a gag. There are times in the interview when Clegg looks so pained as to remind me of Colin Firth in the opening scenes of The King's Speech, particularly when issues of Rupert Murdoch and phone-hacking come up. I know what he'd have said if he were in opposition. The Lib Dems were always very critical of the Cameron-Murdoch cabal. Some Lib Dem MPs were victims of phone-hacking by the News of the World.

“My thoughts are," he begins haltingly, "that it has all come out much more into the open since the police investigation . . . and I think, you know, since those days it is becoming much more out there, and quite rightly. I've always said that the police have got to investigate and the CPS [Crown Prosecution Service] have got to take action. Look, I don't follow every twist and turn . . ." His press secretary looks up for the first time.

What of those, such as the Labour MPs Chris Bryant and Tom Watson, who believe that the Murdochs have too much power and influence over politicians? There's a long pause. "I think that the days when newspaper barons could basically click their fingers and governments would snap to attention have gone," he says.

Clegg is exceptionally loyal to David Cameron – I expect he is a loyal man by nature, not design – but there's a fine line between being loyal and sounding plain disingenuous. So, what does he think of the dinner party hosted over Christmas by News International's chief executive, Rebekah Brooks, at her Cotswolds home, attended by the Camerons and James Murdoch?

“I don't know anything about Oxfordshire dinner parties," he says. Of course he does. Everyone in politics knows about the get-together of Brooks, Cameron and Rupert Murdoch's son, and most agree that the timing of it was inappropriate, given that there was a criminal investigation under way over phone-hacking in the Murdoch empire, as well as ongoing negotiations with the regulatory authorities over the ownership of BSkyB.

“Well, I'm assuming that they weren't sitting there talking about News International issues," says Clegg. "Look, you're putting me in a very awkward spot. If you've got an issue with it, speak to Dave. I don't hang out in Oxfordshire at dinner parties. It's not my world. It's never going to be my world."

He looks pained. I feel sorry for him and I can't help telling him so. I was married to a politician and I remember the constant self-censorship and, in my case, the gaffes. I get the impression that Nick Clegg is an honest, straightforward man in a dishonest, unstraightforward world, in which nobody can say what they really think.

An interruption offers some blessed relief. A beaming middle-aged woman who has spotted Clegg on the train passes a note to his aide. It reads: "I couldn't resist such a unique opportunity to say, 'Stick With It!' The vast majority of us think the coalition are doing the right thing. We know it's tough but it's very necessary. All the best."

The press secretary looks triumphant. Clegg looks momentarily less beleaguered. He thanks the woman graciously and just as I am wondering if it was a set-up, Clegg jokes that it was. He often gets support from the public, he says, but the difference is that these days people whisper their congratulations, "as if it's a guilty secret saying anything nice about Nick Clegg". He should watch those slips into the third person – an early sign that a person is losing touch with reality.

Clegg was a strong opponent of the war in Iraq and for that he earned many supporters. His backing of the "surge" and British forces' continued presence in Afghan­istan is therefore surprising. There are rumours, which he denies, that he wanted to call for an immediate withdrawal of troops but that the former Lib Dem leader Paddy Ashdown, an ex-marine, persuaded him not to.

“In a sense," Clegg says, "we have brought our ambition to a much more realistic level. We've now got an exit date, which we didn't have before, and a much better set of weapons on the ground. And crucially you've got the British government saying to [President Hamid] Karzai – who I had dinner with recently – this cannot be won militarily. Once you're in that far and you've had that many people die and be maimed, I think it would be morally questionable to cut and run overnight."

It is hard to avoid the conclusion that the real reason we continue to pour money into a war with no clear goals – and continue to line the roads of Wootton Bassett – is so that those in power will be able to keep on claiming that "they did not die in vain".

“Look, it's never perfect. It's not a neat world," says Clegg. He is above all a pragmatist for whom coalition, foreign policy and life are a balancing act. He accepts that there are moral problems with supporting Karzai's government, which has no authority outside the Afghan capital, Kabul, and which, according to the Transparency International corruption index, was last year the second most corrupt in the world. "Exactly – that's where it gets messy and imperfect."

Clegg is pleased to have "got more balance into the debate on Israel in the party". While he is "undimmed" in his criticism of Israel's illegal settlement activity and his "absolute horror of what is a humanitarian catastrophe in Gaza", he stresses that "Israel has legitimate security issues in a region where there is a threat to its existence".

He denies that there is a fundamental incompatibility between the west's rhetoric about democracy and our need for oil. "Do we have vital economic self-interest to keep lights on? Yes. Do I think that should be won at the cost of always being on the side of people who want to express themselves and want democracy? No."

He refuses to be drawn on whether he thinks it was bad timing for Cameron to tour the Middle East on a "UK trade mission"- a euphemism for peddling arms to despots – at a time when there are widespread protests in favour of democracy in the region. He will say, though, that the business of selling arms represents "a horrendous dilemma".

That we have sold arms to repressive regimes – tear gas grenades to Bahrain, armoured personnel carriers to Saudi Arabia, crowd-control ammunition to Libya – is "of course wrong", he agrees. "That's why we've suspended scores and scores of export licences. What guarantee do you have when you export product X to country Y, who seem totally hunky-dory, totally peaceful, and what happens when the country goes belly up? What we're doing is pragmatic rather than pure."

Even the language Clegg uses is moderate and qualified, interspersed with phrases such as "kind of" and "on the other hand" as well as rhetorical questions and unfinished sentences. He's unhyperbolic and ambiguous in a way that must be alien to most Tories. Whereas Cameron strikes me as a man with almost no self-doubt, Clegg seems more self-questioning and less bombastic. I suspect that he is as accom­modating and good at compromise in his marriage as he has been politically.

He smiles for the first time when he tells me that his Spanish wife, Miriam, has "got wonderfully strong opinions". It's clear for a start who chose the names for their three children, Antonio, Alberto and Miguel Clegg. They are being brought up as Roman Catholics, even though Clegg has said he is an atheist. The children are bilingual, speaking both Spanish and English fluently.

At one point, it was assumed that Miriam would be the one with the big career and he would be the thinker and take care of their children. After his eldest son was born, Clegg says: "Miriam was in a particularly intense period of her career and I was in a particularly relaxed period of mine . . . coming to the end of my time as an MEP, so I was very, very involved. I wasn't the primary parent – Miriam would get very annoyed if she were to read that – but I was very involved and you carry that on with you."

He has successfully managed to keep his family out of the spotlight, "to create a firewall" between his world and theirs, although he worries constantly that "what I am doing in my work impacts on them emotionally, because my nine-year-old is starting to sense things and I'm having to explain things. Like he asks, 'Why are the students angry with you, Papa?'"

Clegg refuses "to play politics" with his children, or to say whether or not they will go to a private school. While he's not "ideologically opposed to fee-paying schools existing", he is offended by the notion that it would be his decision alone, rather than one he would reach with Miriam. "I go: hang on a minute – what century are we living in?"

The same applies to what he might do in the future. He certainly does not want to be in politics all his life. "I think that's deeply unhealthy. I look at those people that got into politics when they were 16 and are still at it in their late sixties and think, 'My heavens above!'" Judging by the most recent opinion polls, he may not have the luxury of choice. Either way, he says, Miriam has made "masses of sacrifices putting up with me and politics" and this will be something they decide on together. He'd like to think, though, that he would go into education.

He is besotted by his "three lovely boys" and is most proud "by a long shot" of the family life he has created with Miriam. They manage to lead a relatively normal life, "not in a bunker in Westminster", and he tries to pick his children up from school and put them to bed at night at least two or three times a week.

He regrets that sometimes he doesn't always get the balance right, which makes him "quite miserable" and unable to do his job properly.Sometimes he has to tell them white lies if he is stuck in a meeting. At home, in the evenings, he likes to read novels and says he "cries regularly to music."

I receive a snapshot of his family life when, after the interview is over, I am invited to dine with other journalists at Chevening, the grace- and-favour house in Kent that Clegg shares with William Hague. Clegg arrives two hours late – he's been in protracted discussions over Libya – and looks corpse-like with exhaustion. The contrast with his vibrant, pretty wife, with her big bawdy laugh, could not be more stark. His children seem delightful – and delightfully normal.

Clegg has been accused of selling out, of providing a yellow fig leaf for the Tories' less attractive bits. But I expect that he would see opting out of the coalition or leaving politics altogether as the biggest cop-out of all. He is not consumed by politics – he has a fulfilling life away from Westminster – but he seems to have an old-fashioned sense of duty and believes that, without him there in the cabinet, the Tories would be up to far more of their old tricks. He might well be right – but will he be so easily forgiven by the voters?

“I have a faintly romantic belief that if over five years I just keep steadily trying to do the best I can, with all the difficult dilemmas we face, with not very much money, all those kinds of things . . . we will kind of come through. I think if people see that someone is trying to do the right thing and maybe they're not entirely succeeding, they kind of will go with you. And that's all you can do."

He suddenly looks very, very sad. A week later I glimpse him on television, on the front bench on Budget Day. Cameron sits to his left, looking ruddy and shiny, straight off the playing fields, ready for an interminable life of "Yeah, yeah, yeah" in the Commons. Clegg, by contrast, looks like he's in black and white – lost and out of place.

Later that evening, I get a text from his press secretary, offering me "a full copy of the note that lady passed on the train". He thought I might like it for my piece, "in case it needs some colour".