Tim Farron is on home territory and it is hard to keep him moving. He has hardly set foot outside the door of the Liberal Democrats’ office in Kendal when a local councillor waves him down. “Did you get your credit card back, Tim?” he asks. Farron left it in the car park ticket machine the day before, when preoccupied with thoughts of the general election on 8 June. “Yeah, great, thanks, got it,” Farron says. A bit further on he bumps into the editor of the local paper, the Westmorland Gazette, and more chat ensues. The clock ticks on and his spin doctor, Paul Butters, strides ahead, trying to force the pace. We stall repeatedly along Finkle Street, the lakeland hills in the distance, as Farron engages elderly ladies in light conversation. This is his own seat, a safe Lib Dem one, and Butters wants his boss to get a move on – and get out of town.

For the Lib Dems, this election will be different. The strategy is to venture beyond the party’s comfort zones, to seek out potential converts who voted Remain last June. Like Star Trek’s Captain Kirk, Farron’s task is to boldly go where no Lib Dem leader has gone before, or at least where no Lib Dem leader has ever thought victory was possible before.

So we head two hours down the motorway to Manchester Gorton, a previously safe Labour seat. “It could soon be ours,” he says, as we enter the constituency. That is some thought, given the Lib Dems secured just 4.2% of the vote here in 2015, coming fifth behind the Greens, the Conservatives and Ukip. Labour took Gorton at a canter with a majority of more than 24,000. But last June the vote for Remain here was more than 60% and that, the Lib Dems believe, could change everything.

When Theresa May called a snap general election last Tuesday, a byelection contest – due to conclude on 4 May – was already under way in Gorton, following the death in February of veteran Labour MP Gerald Kaufman. The signs for the Lib Dems have been positive for more than a week, surprising even the most optimistic activists. “The polling, the canvassing that we have been doing, puts us ahead of where we were at this stage at the Richmond Park byelection [which the Lib Dems took in dramatic circumstances from the Tories last December],” Farron says. “So I think if I was a betting man, I’d say our candidate Jackie Pearcey would have been an MP on 4 May. I think she’ll now have to wait until 8 June. We’ve got these seats that were not seriously on our radar and now they are.” It is a bold prediction.

The Lib Dems are hoping to mount a new challenge as the non-Brexit party in seats like Manchester Gorton. Photograph: Christopher Furlong/Getty Images

New targets – with little previous Lib Dem history – are now dotted across the country in areas where no one would have thought the party had a prayer a few years ago. Some are held by the Tories with big majorities like Esher and Walton in Surrey and St Albans in Hertfordshire. Others, like Vauxhall in London, the seat of anti-EU MP Kate Hoey, were comfortable Labour wins in 2015. The Lib Dems have their eyes on all of them.

The party suffered an electoral collapse two years ago, after five years in coalition with the Tories, losing 49 seats. For many Lib Dems it seemed impossible to chart a way forward the morning after, when they were left with a rump of just eight MPs. “We had to be prepared for it [recovery] to take a long time,” Farron admits. But victory in Richmond took them up to nine and it was a start.

Farron is part the optimist, part the cautious leader. He refuses to say how many MPs he hopes to have in seven weeks’ time. “More than nine,” is his stock reply. But he can see a big opportunity. Two factors have combined to give him and Lib Dem activists real hope. The first is the weakness of Jeremy Corbyn’s Labour, which he describes as “the worst opposition in human history”. His aim is to replace Labour as the biggest party of opposition in parliament.

The second is Brexit, which after 23 June last year, created a huge pool of disaffected Remain voters – the 48% – in which pro-EU Lib Dems could fish for votes. A gap opened in the market. Clarity of message was, above all, what was needed to maximise new recruits from the Remain ranks. At the heart of Farron’s offering is a second referendum on any eventual Brexit deal.

The Lib Dems will also campaign for the UK to stay in the single market. Even Tories did not want to leave it before May came along, he says. “It is Margaret Thatcher’s legacy. May has no mandate to take us out of it.”

His lines are clear on Brexit and the need for the country to have a proper, united opposition. “If you want to prevent hard Brexit, want to prevent us leaving the single market, if you actually want to give the British people the final say on the terms of the as yet unknown deal, which of course would allow people to vote to remain should they wish, and if you want actually a decent, proper opposition party in this country, then we have this wonderful opportunity of an incredibly clear message that nobody else has. Those people who were on the losing side need someone to speak for them.”

Farron was critical of Jeremy Corbyn’s leadership of Labour, and ruled out a coalition. Photograph: Danny Lawson/PA

The new approach also involves a huge shift in attitude to coalition government, which he reveals in our interview. There will be no coalition with either Tories or Labour after this election, whatever the result. “There is no way we can countenance any kind of arrangement or coalition with the Conservative party and likewise with the Labour party led by Jeremy Corbyn,” Farron says. “He accepted hard Brexit, he voted for it.” It is clear he does not want to put off Labour Remainers who fear a deal with the Tories, or Tory ones who would reject a hint he might team up with Corbyn. But hadn’t Farron voted to go into a coalition with the Tories in 2010? “I did,” he says, adding that circumstances might be different in future. This time, though, “we will not be doing any king of deal, any kind of coalition, any kind of arrangement”. What did Nick Clegg, the former leader, think of his new approach, a policy that would bar Lib Dems from power and from accepting ministerial posts, in the unlikely event the results offer no party an overall majority? “He was at least as enthusiastic as I was” when they discussed this approach, he says.

It is not just Brexit that the Liberal Democrats will talk about on doorsteps over the next few weeks. Farron believes his party now occupies the sensible middle ground abandoned by others in the disturbed political circumstances of the past year. “We believe in really strong public services, well-funded from fair taxation. We believe that climate change is real. We believe in free trade, in working with our neighbours [internationally]. A year or two ago, that was all motherhood and apple pie. Now we are actually the only people who believe it any more. We’ve got to a stage where being sensible is utterly unique and radical.”

He is clearly pumped up as we arrive at a Manchester park, where a gaggle of Lib Dems, holding aloft “Lib Dems Winning Here” banners, await the leader. It does not look like the most natural coming together of Liberals ever. It is one of the many staged events that Farron will attend before 8 June. In his first general election as leader, he knows he will come under intense scrutiny. “I’m aware that a lot rests on my shoulders.”

He draws strength from his Christian beliefs. He does God, unlike Tony Blair, but tries to keep it separate from politics. He attends church every week. Does he pray every day? “Err … Christians pray, that’s what you do.” The strength of this faith, and his parliamentary voting record (he abstained on the equal marriage bill’s third reading in 2013) have led to questions about his attitude to homosexuality, which again he declines to shut down once and for all. When asked, as he has been before, whether he thinks gay sex is a sin, he offers only half an answer. “I mean I do not believe being gay is a sin.” The question is put a second time and he almost ducks it altogether. “We are a secular society in which people should keep their religion and their political pronouncements really quite separate, which I think Theresa May has not done by the way.”

Farron’s political lines are clear and by the end of our interview we know where the Lib Dems stand on coalitions and Brexit, Labour and the Tories. But some of his private views will clearly remain private. I wonder, partly as a result, if anything about the election frightens him? Not a bit of it, he says, getting back on track. “It is a great opportunity, to give Britain the chance to change its future. What’s not to like? What’s not to be excited about?”

Join us at the Convention on Brexit

Ten days into the general election campaign and four days after the result of the French presidential election, the Convention on Brexit and the Political Crash will be held in Central Hall, Westminster, on 12 and 13 May in association with the Observer and OpenDemocracy.

The big questions of this election – the NHS and the economy – will be at the centre of the agenda, but there will be much else. How does Britain fit into the new Europe? What will the French election bring to the future of 500 million European citizens? As the far right surges across the continent, could the UK’s best plan be offshore detachment?

And who are the British now? There will be key sessions on Northern Ireland and possible union with the south, and on Scotland, where the election is seen as a test for another independence referendum. Is the UK disintegrating and do the English care? One big question is whether the election will heal the divisions of the referendum. Will it end the rise of racist and sexist abuse online and in the media? Indeed, what of the media in the post-truth age?

Contributors include Ian McEwan, Caroline Lucas, Alastair Campbell, Charles Grant, Lisa Nandy, Harriet Sergeant, AC Grayling, Anne Applebaum, Michael Gove, Ian Dunt, Douglas Carswell, Mary Lou McDonald, Jay Watts, Bob Geldof, Fintan O’Toole, Vonny Moyes and Anand Menon.

Observer readers’ ticket special offer for the two days is £35 using the code OBSERVERDISCOUNT at theconvention.co.uk