Politicians' nicknames are seldom flattering, but the joke going round Westminster for the last year about "Invisible Clegg" had seemed particularly cruel - chiefly on account of ringing true. When we met a fortnight ago, at Waterloo station, I'm pretty sure I was the only person on the concourse to recognise him - and on the train journey to his home in south-west London, he drew not so much as a flicker of interest from other passengers. For the leader of a third party struggling to make an impact, invisibility is an ominous quality.
Clegg had never allowed access to his family home before, but from what I saw he could give WebCameron a run for its money when it comes to cuteness. Deep in leafy Putney, the front door opened on to a vision of middle-class domesticity - a newborn baby dozing in a basket, and two young boys prancing about in their pyjamas, the elder proudly writing out his diary for the week: "school and French" one day, "church on Sunday" and so on. Clegg's Spanish wife, Miriam, a lawyer, offered beer and smiles while Clegg momentarily lost himself in his children - "Hello, monkey!" - prompting a gurgly smile from his youngest.
We moved into the conservatory, and the boys clambered all over him, tugging and playing as they scampered in and out. When one of them fetched homemade snacks from the kitchen and carefully handed them around, Clegg almost blushed. "I have to say, this is quite atypical!" The younger boy came charging in to announce the arrival of the photographer - "It's someone who's making photos!" - and his father melted, then looked briefly thrown. "I'm finding this actually quite - well, I've never been interviewed before at home, and I feel slightly ..." Self-conscious? "Yes, I do. I do, actually. I'm not used to this at all."
As he outlined his vision of a Liberal society, his eldest son held up a piece of A4 paper on which he'd painted "Blah blah blah blah blah" - and I had to laugh. It wasn't a bad critique of Clegg's leadership, which had until then seemed to lack a certain killer instinct.
Clegg is much more animated in real life - more colloquial, less plummy - than he often appears on TV. People say he looks like David Cameron, but in fact he looks uncannily like a younger version of the Dutch football coach Guus Hiddink. When he gets worked up, he's prone to drumming the table - not a great thump, but a solid thwack - and his voice frequently leaps several octaves. He gets particularly exercised about our "crackers" maternity leave laws, which he says institutionalise inequality between parents, both at home and at work. "It's insane!" he exclaims.
It has often been said that Clegg has a slight issue with his temper - commentators like to say he "flushes" easily, and I saw a few glimpses of testiness. But in truth it seems more like impatience than irritability; a multilingual Cambridge graduate, he is certainly cerebral, which may account for some of it. "I sometimes want things to go much faster than they do," he agrees ruefully.
The bigger problem had seemed to be a tendency to somewhat abstract intellectualism. He repeatedly quotes the Liberal party's old slogan of "conscience, reform and progress", which are all admirable ideals, but possibly not the sort of soundbite to grab headlines in an age of 24-hour news. His predecessors managed to distil a reason to vote Lib Dem into a simple idea - opposition to the war in Iraq, or student tuition fees; a penny on income tax to fund education - yet Clegg's big idea had seemed to elude him.
When we met, the revelations of expenses claims had so far been less damning for the Lib Dems than their rivals, and he had just forced the first Commons defeat of Brown's premiership, over the Gurkhas. Yet over the conservatory table, he told me, "I have this very old-fashioned idea that yes, politics is about scandals, it's about who's up and who's down. But actually, it's about the battle of ideas, right? Ideas about what is the best society." Later, I overheard him editing with an aide an article for the Observer about Michael Martin, and was slightly concerned to hear him saying things like, "I don't want to sound too shrill," and "Boasting 'I did this, I did that' sounds too self-serving." If he's not going to seize this moment with both hands, I thought to myself, when will he ever?
But if a week is a long time in politics, a fortnight is an eon. Clegg's Observer article broke ground as the first call by a party leader for the Speaker's head - which duly fell within days. The government conceded the argument over the Gurkhas. One of its own senior ministers, Alan Johnson, has called for a referendum on proportional representation (PR), and last week Clegg launched his own 100-day plan for radical reform, taking hold of the agenda. For the first time since becoming leader 18 months ago, he suddenly looks not invisible at all, but like a force to reckon with.
This is, he agrees when we speak again late last week, the best chance for PR he has seen in his political lifetime - "without a doubt". The question now is whether he can take it.
There is a certain irony that, until recently, Clegg had more or less abandoned the idea of campaigning for PR. "When I became leader I looked at what had happened, with Ashdown and Blair - and the party getting its knickers in a twist - and I made my mind up very early on that you don't change the system by waiting for one of the vested interests to throw a dog a bone," he told me at our earlier meeting. "We've been there, I've seen enough of my predecessors being led up the garden path and then disappointed.
"And for very good and simple reasons. Why on earth, if you were Labour or Tory leader, would you say 'Yeah yeah yeah, change a system that has benefited us massively, and enabled us to govern with no mandate and almost no scrutiny or checks and balances.' Of course they're not going to give that up! And if they say that in opposition, they never do in government. So it is a very, very pragmatic simple insight that we're not going to get PR by jumping up on the sidelines saying I want PR! I want PR!"
Except that now, this is precisely what he is doing. So what's changed?
"I think because the anger has been so sustained over such a period of time, and the feeling of crisis has become so all engulfing, that for the first time in a generation it's been possible for electoral reform to get a voice. That was only possible because of the sustained public outcry. People feel this isn't a few rotten apples in the barrel; the whole system is rotten to the core."
It's a delicate line between harnessing anger and looking opportunistic. If Clegg has capitalised shrewdly on the crisis thus far, the question is what he does next if the main parties simply choose to ignore him.
"I've got cards up my sleeve," he says coyly, but won't elaborate. Instead, he offers, "Let me spell out what I think is going to unfold. There's clearly something unsustainable about MPs saying, 'We've done something seriously wrong but we are going to hang around until the next general election.' I think for the most serious abuses, instead of having MPs adjudicated by party panels behind closed doors, the adjudication will have to transfer to independent hands such as the standards commissioner. Then I think the demand for byelections will be overwhelming. If we don't have a general election we're going to have a war of attrition. People won't be satisfied by vague ideas. They're not going to be fobbed off with that."
But surely a general election now would be the worst possible outcome for the Lib Dems? The Tories would be likely to win, and all momentum for the sort of reforms Clegg wants would collapse.
"Of course I want a general election! I think if a general election was held now, it would become an all-out national debate about how we rebuild British politics. The next general election will be dominated by: Who's going to rebuild the British economy? And who's going to rebuild British politics? I'm very excited, because I think we've got really compelling things to say on these two big-ticket items. It's now for the Lib Dems to be slap-bang in the vanguard of the debate."
But what, other than electoral reform, does Clegg stand for? As with many Lib Dem leaders before him, the answer hasn't always been clear. Since his election to parliament in 2005, he has been closely associated with the Orange Book wing of the party, leading it away from tax-and-spend towards a lower-tax-and-smaller-state philosophy. He characterises it as classic Liberalism, but has been accused of representing Cameron-lite - a charge that struggles to stand up when he talks with un-self-conscious passion about civil liberties, internationalism and, above all, social mobility.
"Sheffield, where my constituency is, is a city where if you're a child born in the poorest area, you will die on average 14 years before a child born in the wealthiest neighbourhood. You've got totally grotesque levels of inequality which haven't shifted at all under Labour. Not at all. I'm not saying it's for lack of intention; I actually believe in the good intentions of a lot of people in the Labour party when it comes to social mobility. But I completely question their ability to actually do it."
I spend a day with Clegg in his constituency, where he meets the members of a new Community Justice panel, a Lib Dem initiative designed to divert young and minor offenders away from the "revolving door" of the criminal justice system. In its modest way it is profoundly inspiring; precisely the sort of progressive initiative that Labour voters in 1997 had hoped to see. It is these voters who Clegg is now explicitly chasing - but if they're disillusioned with Labour, what can Clegg offer that Cameron's hoodie-hugging, broken Britain-fixing Tories can't provide?
"Do you think," he snaps back, "that the Conservative party as an establishment party is going to take on the vested interests in a tax system? Is going to say to middle-class folk that little Johnny will be in a class with another child who will have more money allocated to them? Are they going to do that? No. Are they going to take on the Daily Mail on Europe? No. You're never going to get that from the Conservative party. Never. You'll get it rhetorically, you'll get the spray paint - and it's a great spray job he's done already. But you won't get the conviction."
In all our conversations, Clegg says nothing that could have come from Cameron's mouth. There is little discernible similarity as a character either; any confusion between the two men would require, for one thing, almost total ignorance of our class system, for Clegg's Eurocentric internationalism - his mother was Dutch, his home life is bilingual, he used to work for the European commission - places him on a different planet from the sloaney Englishness of the Tory leader. He is a quintessential Liberal, and on issues from ID cards to the environment can plausibly claim to be ahead of the curve. But even his stated ambition to double the number of Lib Dem MPs - currently 63 - within two elections still feels ambitious. Does Clegg seriously believe he will one day be prime minister?
"Yes!"
I ask him why he's smiling. "I'm smiling because I know what's coming next." He's not wrong; what I mention next is his party's poll rating. It's pretty static, I suggest, isn't it?
"No it's not! It's jumped four points this week!" he exclaims indignantly.
But it's not part of a steady upward trend; it looks more like a ceiling.
"Look, let's speak after the next general election, OK?" he retorts, looking huffy. "Let me just try my bit on some of the psephologist stuff. Our poll rating is far, far higher than it's ever been at this point in the political cycle. We always tend to go right, right down, particularly about a year from a general election. I was warned privately by pollsters last summer, don't worry if at this point you're down to 11%. In fact we're at 22%!
"In the south the only battle is Conservative/Lib Dem, and in the north it's Labour/Lib Dem. Don't you think it's quite significant that the only party that has a national spread now, south and north, rural and urban, is the third party?" But they are in second place, I point out, in all of them.
"Yeah, but we're second place in an environment where we've made massive gains. You know, I ask people, do you think it would have been possible four years ago that Sheffield would have been a Liberal Democrat city? Who would have thought that Newcastle would be run by the most astonishingly popular Liberal Democrat administration? Things are really changing."
It is the fate of Lib Dem leaders to trumpet these local incremental gains, only to be disappointed at the polls. But having been eclipsed by his deputy Vince Cable for so long, Clegg is emerging as the party's most promising leader, and knows that opportunities like this one don't come along very often.
If this has been his best month as leader, I ask what has been his worst moment.
"Well, I don't think my interview with Piers Morgan was probably my greatest moment," he offers wryly. His notorious claim to have slept with "no more than" 30 women was more or less the only memorable statement he made in his first year in charge. What on earth, I laugh, had he been thinking?
"Well look, I've actually never talked about this before. I couldn't say anything about it straight afterwards, at the time, because everyone was laughing at me - quite rightly. Yeah, you know, I would as well. But what I was doing was interrupting him as he was making up numbers. Of course I wouldn't make a claim like that! And suddenly it's hung round my neck. But there's no point saying it at the time - and, of course, I've only got myself to blame."
What did his wife say to him? He shrugs and grins.
"Oh, you know, of course it's not great, of course it isn't. But I've learned the hard way, haven't I?"