After Eastleigh the Lib Dems still have a long way to go, but don't bet against them just yet

Inside Westminster: Clegg feels he has proved the critics wrong and allies say he is now 'absolutely secure' in his post for the 2015 election

"We will fight 57 by-election campaigns at the next general election," smiled the Liberal Democrat strategist as Nick Clegg's party breathed a deep collective sigh of relief after avoiding defeat in the Eastleigh by-election.

Jubilant Lib Dems dared to raise the prospect of not merely holding the party’s 57 seats in 2015, but even seizing some Conservative-held marginals.

A week before the Eastleigh contest, the Lib Dems were confident of a comfortable victory. Then they were destabilised by the untimely allegations that Lord Rennard, their former chief executive, sexually harassed party activists. To say there was a wobble about the Eastleigh result is an under-statement. “Our canvassing figures were all over the place; it was impossible to call it,” one Clegg aide admitted.

Victory allows Mr Clegg to feel vindicated. His internal critics doubted the party could win while it is in power. Although local issues played a big part in the by-election, the Lib Dems also fought on their record in the Coalition. Mr Clegg feels he has proved the critics wrong, and allies say he is now “absolutely secure” in his post for the 2015 election. The party is good at digging in once it wins a seat, and projections based on nationwide opinion polls do not take account of that. Incumbency is a bigger factor now that MPs have become glorified councillors, no longer able to ignore constituency casework.

The Lib Dems deserve plaudits for sticking to their guns and Mr Clegg has shown resilience under fire. But the party leadership shouldn’t get too carried away. At the general election, the Lib Dems will not be able to flood all their 57 seats with the hundreds of party workers they dispatched to Eastleigh; there will not be enough to go round. And Mr Clegg’s party may struggle to repeat its performance in Hampshire in the north of England, where voters may be less forgiving about the Lib Dems jumping into bed with the Tories in 2010. Labour stands to benefit.

Defeat in Eastleigh would have provoked a leadership crisis for Mr Clegg. There are many in his party who believe he is so “toxic” in the voters’ eyes that he cannot be “decontaminated”, that the party is so damaged it cannot be repaired while he remains at the helm. After Eastleigh, his critics will have to bide their time. But it is wrong to assume that Mr Clegg is with one bound free to lead his troops into battle in 2015.

Some Lib Dem members fear the party could come fifth behind the UK Independence Party, Labour, the Tories and the Greens in next year’s European Parliament elections. Mr Clegg could then face a grassroots revolt aimed at installing Vince Cable as leader, while allowing him to remain as Deputy Prime Minister until the 2015 election.

In the short term, it is David Cameron who is under pressure. The Tory MPs who cannot forgive him for failing to win a majority in 2010 are even more convinced he is a loser. The demands for a tax-cutting Budget, an-ever tougher line on Europe and immigration and a shift to the right will grow, though he would be unwise to pander to his critics. Having a right-wing candidate in Eastleigh didn’t do the Tories much good.

There won’t be an immediate Tory leadership crisis. Mr Cameron’s internal critics had already priced in defeat in the by-election. But some detect a “slow burner” that could explode later – Ukip’s strong second place. If it comes top in the Euro elections, which looks likely after Eastleigh, Tory backbenchers in Con-Lab marginals will fear for their seats – not that Ukip could win them, but that it could take enough votes to let Labour in through the back door. Although we can expect lots of talk about anti-Cameron plots and leadership coups, I suspect it will come to nothing before the general election. The Tory manoeuvres will really be about what happens if the party doesn’t win a majority in 2015.

To combat the Ukip threat, we can expect a pretty crude scare campaign from the Tories. They are convinced that people will vote differently at general elections and so will warn that voting Ukip could install Ed Miliband as prime minister. This has dangers for Labour. Although it plays down its fourth place in what officials describe as “our 268th target seat”, Eastleigh highlights two big challenges for Labour – how to win in the south and to convince voters it is fiscally responsible. They are not unrelated. Presented by the Tories with a straight choice between Mr Cameron and Mr Miliband, many wavering voters may opt for the devil they know if they don’t trust the other one.

We now know the Lib Dems cannot be written off and will still be the game. Indeed, they might well still hold the balance of power in a second hung parliament. There’s a long way to go yet, but I wouldn’t bet against it.