Back when they were virgins, the Liberal Democrats liked to present themselves as the Goldilocks party. They were not too hot and not too cold, the movement of the happy medium. So it would be fitting if the two radically opposed views that now exist about the party's prospects – that they will be scattered to the four winds in 2015 or they will survive the blast – could somehow be bridged. How very Lib Dem it would be if there were room inside their tent for both Cassandra and Pollyanna.

This weekend, as the party gathers for its spring conference, the doom-mongers will have much to seize on. The latest polling from Lord Ashcroft suggests fewer than a third of those who voted for them in 2010 have stuck with the party: the 23% of the national vote the Lib Dems bagged three years ago has now slumped to 8%. No wonder party president Tim Farron warns the faithful they "shouldn't assume [their] survival is guaranteed". Activists meeting in Brighton need only open the papers, bursting with disobliging revelations about the crises of the Chrisses, Huhne and Rennard, both of which raise awkward questions about what the leadership knew and when they knew it.

The pessimists point to a more enduring problem for the Lib Dems. Crudely put, voters who like the government in 2015 will vote Tory; voters who dislike it will vote Labour; and there will be next to no reason to vote Lib Dem. They will be squeezed until they cannot breathe.

By way of rebuttal, the optimists excitedly tell you they now have an Ashcroft or two of their own – newly recruited, deep-pocketed donors who have funded research showing the Lib Dem core vote is holding steady at 13%, with a decent chance of rising to 20% at the next election in 2015.

These two positions would seem to be irreconcilable: either the party's vote is going to crater in 2015 or the Lib Dems will retain, more or less, their current presence in Westminster. They surely cannot do both. Unless, bizarrely, they can. Think of it as the Eastleigh effect. The Lib Dems were battered nationally, mired in scandal, their poll numbers in the tank, and yet last week they won a closely fought byelection. What if the Lib Dems fight 2015 as 57 Eastleighs, each one a local contest fought as if sealed off from national trends, decided by the party's distinct blend of on-the-ground organisation and dominance in the town hall? Nationally they may be wiped out but, thanks to an electoral system the Lib Dems have opposed forever, it wouldn't matter. They might keep the 9% of Westminster's 650 seats they have today after winning, say, just 9% of the national vote. That would be proportional representation of a very exact kind – and both Cassandra and Pollyanna would have been vindicated.

Imagine too the wider picture. With each passing day the Tories look less likely to win the overall majority that eluded them in 2010. If the Conservatives couldn't win one then, facing a tired government, led by an unpopular PM following a financial crash, how could they win one in 2015, as incumbents blamed for an austerity programme that has been all pain and no gain? Nick Clegg is brutally clear on this point, telling colleagues, "If the Tories can't win in Eastleigh, in the south, with all that scandal, then they're not going to win."

For this, Clegg himself can claim some credit. Outraged by the Tories' failure to deliver on Lords reform, the Lib Dem leader withdrew his promised co-operation on boundary changes, depriving the Tories of perhaps 20 extra seats. With Ukip threatening to divide the centre-right vote, Tory prospects for 2015 get only gloomier.

Of course, Labour may win an overall majority of its own, but few would bet on it. Which leaves the Lib Dems contemplating a return to Westminster having been rejected by millions of voters, yet still retaining the balance of power. Despite the conventional wisdom that coalition always ends up devouring the smaller party, it's just possible that it will be Clegg who stays on the bridge at the start of a new Lib-Lab coalition, waving goodbye to Cameron as he steams off into opposition.

Ah, but would it be Clegg? The leader has been deemed a dead man walking for so long, it can seem heretical even to imagine his survival. The experts say it's only a matter of time before his regicidal party – the knife grown comfortable in its hand after it slayed Charles Kennedy and Ming Campbell – turns on him, probably in late 2014, giving time for the new leader to bed in before facing the voters the following year. But that too now seems questionable.

Eastleigh has bought Clegg time. And who, plausibly, would replace him? Vince Cable's essay in praise of "greatly expanded" capital spending this week was a well-timed reminder to Lib Dems that he stands ready – even if it's a bit rich for him to have seen the Keynesian light now, having served for three years as an enabler for a government whose austerity fetish has choked growth – a fact now embarrassingly confirmed by the Office for Budget Responsibility. But Cable will be 72 in 2015: that may be a tad too experienced for a party that ousted Campbell on grounds of excessive seniority.

When Farron looks in the mirror he clearly sees the next Lib Dem leader, but his strenuous tickling of his party's erogenous zone is done so transparently that too many see through it. Besides, the experience of coalition means the bar is set higher now. Party members need to see their leader as a plausible deputy prime minister, and not many reckon Farron passes that test.

Cameron has played a role here too. Tory funding of the 2011 No to AV campaign, casting Clegg as a liar and promise-breaker – for agreeing to come into coalition – ended any warmth between the two leaders. The Lib Dem resents it to this day: his relationship with Cameron is now civil and purely transactional.

The result is that Lib Dem "differentiation" from the Conservatives is no longer a theoretical objective on a strategist's flipchart: now it comes to Clegg naturally. These days, albeit privately, he effortlessly lays into Cameron's betrayal of the green agenda or his "bonkers" position on Europe, mocking the PM's promised EU renegotiation as "spending three years flying to 27 European capitals to get a tweak to the working-time directive in order to please Liam Fox". The more that Lib Dems see Clegg as the attack dog, and the less they see him as Cameron's poodle, the stronger his position will be.

Of course, if the Rennard inquiry leads to Clegg's door, then all this could be moot. But as things stand, there is every chance that Clegg could become the Hans-Dietrich Genscher of British politics: a permanent fixture of this country's government, the perennial kingmaker, forever determining who wears the crown.

Twitter: @j_freedland