There is a post-7 May scenario in which Nick Clegg remains Lib Dem leader. It requires five things to happen:

1. He has to hold his Sheffield Hallam seat.

2. The Lib Dems must hold on to at least 30 seats.

3. There needs to be a hung parliament.

4. A coalition deal with either the Tories or Labour needs to be possible.

5. Nick Clegg has to be able to sell that deal to his party and the public.

The first three I'd bet on happening. The fourth is more doubtful: the numbers may just not stack up for a stable two-party coalition in 2015. And the fifth is least likely of all.

Nick Clegg is unlikely to be able to sell a second deal with the Tories to his own, increasingly sceptical, party. As for a Lib-Lab coalition reversing many of the policies implemented by this Lib-Con coalition - how could Clegg credibly even begin to persuade the public?

Even Clegg's most fiercely loyal friends recognise that, if the Lib Dems suffer significant losses and return to opposition, his tenure as party leader will soon end. Which is why attention is beginning to shift to this far more likely post-7 May scenario and the question of who will be the next leader of the Lib Dems.

There's no question who the front-runner is: Tim Farron. His four years as party president obliged him to tour the country, cheering up the coalition-beleaguered activists. He used the position cannily, building a base in the party unencumbered by ministerial office, charting a course of what he would term "independent loyalty" -- never dissing the leader, but voting against £9k tuition fees and the bedroom tax -- yet which critics inside the party label "ambitious expediency".

What even his worst enemy wouldn't be able to attack, though, is his campaigning nous. His Cumbrian constituency, Westmoreland and Lonsdale, had been Conservative for 95 years until 2005, when Farron edged it by just 267 votes, the sole victor in the Lib Dems' ill-fated 'decapitation strategy' that year to oust top Tories. Five years later, he retained the seat with a whopping 60 per cent of the vote. In 2014's Euro elections, it was one of only four areas in the UK where the Lib Dems topped the poll. Farron reaches parts of the electorate where we don't normally get a look in.

For a party grown weary of being constantly on the defensive, looking for a charismatic campaigner to rebuild the Lib Dems beyond the three dozen fortress-seats to which it has retreated, Farron is the natural fit as next leader. Yet the doubts persist, even among those who should be his natural allies on the left-leaning social liberal wing of the party.

Some say he's lightweight, lacking gravitas. "I have the highest opinion of him, but he's not a foreign policy man," said Shirley Williams at the launch of Farron's constituency re-election campaign -- an odd remark to make to a journalist about her own party's shadow foreign secretary.

Farron himself is aware of the charge, and has sought to counter it with a blizzard of lectures and articles on everything from welfare reform to immigration to civil liberties (though housing remains his passion). He has also made a real effort to court the party's economically liberal 'Orange Bookers', contributing to that controversial book's sequel and speaking at conference events organised by free market think-tanks. He might not win them over, but hopes at least to blunt the edge of any anti-Farron campaign.

Small wonder, then, that his nascent campaign has been described as "unstoppable" by Clegg aides. But that will not stop some of his colleagues from trying. The name in the frame at the moment is health minister Norman Lamb, another Lib Dem with a rock-solid seat, who, despite being overlooked when Clegg was first doling out government jobs, has since established himself as one of the Coalition's star acts, successfully pushing overlooked issues like mental health to the top of the political agenda. Lamb's problem is not his ministerial record, but his ability to re-connect with the voters beyond his North Norfolk heartland. As one of his friends bluntly acknowledges, "he's a very sound guy, but comes over as a bit bland."

There are others who could spring forward if either of these two falters. Ed Davey has made no secret that he would like a go, but will struggle to counter the perception he's a bit too middle-of-the-road. Steve Webb, the social liberal who has won plaudits for his economically liberal work as pensions minister, while quite miraculously avoiding party activists' blame for his double-act with Iain Duncan Smith, might fancy a tilt at the leadership (though he didn't in 2007, when his chances of success were higher).

Scottish secretary Alistair Carmichael might present himself as an avuncular unity candidate, despite regular disavowals that he's interested. If Jo Swinson beats the odds to hold her East Dunbartonshire seat, she would stand a very good chance -- on her own merits, but also because the party is embarrassed by its lack of women MPs, and likely to be more so after the next election (though chances are the Farron or Lamb teams will try to sign her up to a 'dream ticket' as deputy leader).

Two oft-mentioned figures don't feature in the list above. Vince Cable is 72 this May and thought himself too old to stand for the leadership eight years ago. And Danny Alexander, even if he beats the odds and retains his seat against the SNP onslaught in his Scottish seat, has been far too closely associated with unpopular Coalition decisions to stand a chance of winning the all-member ballot.

So there you have it: the runners and riders for the Lib Dem leadership. Barring some startling shifting of the plates, it seems almost certain that Tim Farron will be giving the leader's speech at the party's Bournemouth conference this autumn. He will, naturally, use the occasion to pay full and handsome tribute to Nick Clegg. But no-one expects him to lead the Lib Dems in the same direction.