Nominations for the post of leader of the Lib Dems don’t officially close until Thursday, but we already know the name of the winner – Vince Cable. In fact, Cable tells me, as every other MP has pledged to support him, he is already de facto leader, and has just come from a strategy session with his parliamentary colleagues to discuss how to get their ailing party back on the map. In the election in June, the Lib Dems managed to gain a few seats and now number a round dozen, but their vote share was a measly 7.4% compared with 23% in 2010. The collapse has been calamitous.

“One of the advantages of having a coronation as opposed to a competition is that it gives us time to plan and organise,” he says. “There is a downside, but the upside is that I can get on and start doing things, and I already am. I’ve been out there in the past 10 days on Brexit and the economy.”

I congratulate him on his recent interventions – likening Theresa May’s attack on rootless “citizens of nowhere” to the language used by Hitler (though he later revised his opinion and said it was closer to Stalin) and suggesting to Andrew Marr that “Brexit may never happen”.

“We have to be heard above the noise,” he explains. “We’re not the force we were in 2010 in terms of MPs and vote share, and it’s a big challenge to build it back up again.”

Cable, who is 74, hesitated before throwing his hat – he is very much a hat man – in the ring, but claims others encouraged him to do so. If Jo Swinson, the favourite for the job, had stood, he says he probably wouldn’t have opposed her, but she preferred to go for the deputy leadership. He denies rumours that he will stand down after a few years to make way for Swinson, and insists that in the unlikely event that the government runs its full term, he will be ready to fight an election despite being almost 80.

Ironically, 10 years ago he was seen as too old for the leadership – the Lib Dems wanted someone younger to succeed Menzies Campbell, and Nick Clegg got the job. Cable was a contemporary of former Conservative chancellor Norman Lamont at Cambridge. Lamont now seems like a figure from the Early Devonian, whereas Cable is just ascending to the top job. “He peaked too early,” says Cable drily. “I’m in the foothills and still climbing.” Gladstone, as Cable has been quick (if a little vainglorious) to point out, was prime minister in his mid-80s, and that was without regular visits to the gym.

Cable is, in any case, a young 74. I meet him at Waterloo station – under the clock as tradition dictates – and he easily outpaces me in the dash for the 3.20pm to Twickenham, where he is holding his first constituency surgery since regaining the seat with a thumping 10,000 majority in June. The woman he is sitting next to on the train recognises him, says she is a Lib Dem activist and insists on a selfie. He quickly produces a comb to run through the remnants of his hair before she takes the picture. The dapper, ballroom-dancing Cable – today sporting an elegant suit and striking tie, though not his trademark fedora – may be more image-conscious than his Victorian predecessor.

After his defeat in the 2015 election, Cable said he was done with politics. He was enjoying a productive retirement, writing books, taking up academic posts, and working to get a community bank off the ground. But he hadn’t reckoned on a snap election. The local party wanted him to stand again on the grounds that he had “brand value”. He says returning to politics “wasn’t totally free of hard choices”, but you sense he is pleased to be back in the limelight, especially at a time when politics is so febrile. Every crisis is an opportunity, especially for a struggling party.

Facebook Twitter Pinterest Vince Cable at the National Liberal Club, London. Photograph: David Levene/The Guardian

The crisis he has in mind is, of course, Brexit. “This is going to dominate politics for the next few years and we’re very well positioned,” he says. “We weren’t well-positioned a few months ago, people couldn’t quite get what we were trying to say. They thought we were harking back and trying to rerun the referendum, when actually we were anticipating a very difficult process of Brexit and then giving the public a choice at the end of it.”

Cable stands by his suggestion that we may never leave the EU. “The Brexit process is going to get very messy. I meet a lot of senior civil servants and they’re trying to be loyal, but their eyebrows rise. They just can’t see how it can be done. The government haven’t taken on board the complexity of unwinding 40 years of regulatory activity.” He says the row over Euratom is a taste of the chaos to come. “The Brexiteers are only just beginning to understand the enormous can of worms they have opened up.”

Does he see any form of Brexit that can work? “I’m increasingly pessimistic,” he says. He thinks there is a 50-50 chance that Britain will get a deal and transition period that safeguard the economy. “That could happen,” he says. “But I think there’s a very high risk of the whole thing falling apart and [the UK] crashing out, with all the costs associated with it. It’s at that point that the second referendum becomes absolutely essential.”

Brexiteers have warned of riots at the first sign of backsliding. “That’s dangerous, inflammatory language,” says Cable. “It’s just a bit short of inciting violence. There are people on the extreme nationalist right who will react. Whenever I say something, I get a fair degree of trolling, people who are threatening to beat me up. There would a backlash among people who are very fervent, but that’s why you have to have a second referendum. It’s the only way of establishing legitimacy to go back.”

The outgoing Lib Dem leader Tim Farron thought the promise of a second referendum if Brexit turned sour was his party’s trump card, but it flopped at the election. Cable, though, believes it might yet prove a vote-winner. “The potential is enormous. A lot of people who supported Labour [in June] didn’t appreciate exactly where Corbyn is coming from on Europe, and they’re going to get very disillusioned when they do realise it. The Pied Piper of Hamelin led them on a merry route and they’re currently happy enough, but when they realise he’s just as committed to a hard Brexit as Theresa May, they will react very badly.”

He also thinks Labour will eventually crash and burn on economic policy. “This phrase ‘the magic money tree’ has become a kind of shared joke, but that approach to economics is not going to stand serious scrutiny. Economics will come back. It was amazing that it just didn’t feature in the last election.”

He doubts whether Corbyn can win an election. “We all underestimated him. He managed very well and was good on television, but he’s a pro-Brexit leader with an anti-Brexit following, and the Venezuelan economics is not, I think, likely to appeal.”

Cable is convinced that faced with Tory and Labour parties that have committed themselves to Brexit, there is plenty of scope for his party to drive through the centre. The problem with his thesis is that all the factors he cites were in play in June, yet the Lib Dems still underperformed. “It was a missed opportunity,” he says. Does Farron have to carry the can? “I don’t blame him,” says Cable, before going on to blame him. “He got distracted by this moral problem [over gay sex], and wasn’t heard when he should have been heard.”

I ask him what would constitute success for his leadership. “If there is a significant improvement in our vote share, and a sense that we are back in the frame as a serious party being listened to,” he says. He is not looking merely at picking up the odd seat – his age doesn’t permit him that incremental approach – but is more interested in driving up popular support. “You could come from third to first very quickly.” He is encouraged by Emmanuel Macron’s remarkable ascent and the appeal of “radical centrism” in France, and draws parallels with Britain. “The right had become discredited in France, while here the Tory brand is becoming discredited by the day, and there was also a reaction against the Mélenchon left.”

Might there yet be a grand realignment on the centre-left of the sort championed by another of his predecessors, Paddy Ashdown? Cable feels the moment has passed. “There was a lot of talk about this six months ago among Labour remainers, but parties tend to hang together. When you look at what people like Michael Heseltine and Ken Clarke say about their own party, it’s pretty hard-hitting, but you don’t ever get the sense they’re going to walk away. ”

The tuition fees debacle, when the Lib Dems first promised to abolish them but then ended up in government agreeing to them being trebled, still dogs the party. Cable is determined to try to lay it to rest. He thinks Corbyn’s pledge to abolish fees and write off student debt is fantasy economics, but blames the Tories for ending maintenance grants and allowing interest rates on loans to spiral. “I defend what we did,” he says, “but it’s clear the current system is difficult to justify in its present form.”

His big idea is to introduce “learning accounts” – grants for everyone over the age of 18, regardless of whether or not they go to university, to cash in as part payment on a degree or some other form of training, or to be reserved for study in later life. Cable thinks it would be democratic, economically manageable, and would both protect the income of universities and keep down student debt. “We need something bold like that,” he says. By “we” he means the country, but there is no doubt his damaged party also needs it to start to win back the trust of younger voters.

Before such strategic policies can be formulated, however, there is a battle to be waged over Brexit. The big opportunity for an anti-Brexit alliance to form in parliament – Cable is already in touch with Labour remainers and wants to draw in Tory rebels too – will come in the autumn, when it starts to vote on the so-called great repeal bill, published on the day we meet. “There are a lot of problems with the bill,” he says, “not least that it is potentially highly undemocratic to have hundreds of major pieces of legislation passed through on what are called Henry VIII powers, which effectively transfer responsibility from parliament to the executive. Given that the whole purpose of Brexit, as I understood it, was to restore parliamentary sovereignty, this is beyond a joke, and there will be enormous resistance inside and outside parliament to it.”

Can May, who according to Cable is now primarily “a human shield” for her cabinet colleagues, survive this parliamentary ordeal? “I wouldn’t write her off,” he says. “She could come back after the summer recharged. I have noticed that, even though she is getting slaughtered in the press, she is quite confident in parliament.

“I’ve been through this myself as secretary of state [for business] at several points. ‘Oh, he’s finished, it’s all over, he’s going to be out in a week,’ but you weather the storm, you’re back there and you move on.” And Cable is still there, enjoying being back on the parliamentary dancefloor, one of the great survivors in a highly charged political tango.