Winning the Eastleigh by-election provided Nick Clegg with some desperately needed credibility just in time for the Liberal Democrats' spring conference.

He made the most of it in a speech designed to unify the party but clinging on to a seat where they have a solid track record in local government and Ukip split the Tory vote will not be sufficient to boost the party's prospects or his own as leader.

Mr Clegg faces scepticism among his own members as well as from the electorate in general over whether he can deliver on his key conference messages by the 2015 General Election. The claim that the Liberal Democrats are no longer a party of protest but a party of government is a double-edged sword. As coalition partners supporting the Conservatives' austerity plan, they have alienated supporters who believe the required reform of the welfare state has been a Trojan horse for unjustified benefit cuts.

The economy, the deciding issue of the next election, is one that divides the party. Vince Cable, the business secretary, has been increasingly critical of the Chancellor's reluctance to borrow more to invest in infrastructure projects to boost growth. Mr Clegg, in describing this Plan B approach as a myth, will risk losing the left of the party by aligning himself with George Osborne's austerity programme, despite acknowledging more must be done to mobilise investment into our long-term infrastructure needs.

While some of the embarrassment over the expected jail sentence for former energy secretary Chris Huhne has been assuaged by victory in the by-election caused by his resignation, the ineffectual response to allegations of inappropriate behaviour by the party's former chief executive, Lord Rennard, must lead to questioning of its commitment to the practice as well as the principle of equality. This sense of reneging on a fundamental liberal value will gain traction with those who see the party's support of secret courts for terrorism trials in England and Wales as breaching its stance on human rights. Mr Clegg should be grateful to the home secretary, Theresa May, for the suggestion that a future Conservative government would pull out of the European Convention on Human Rights since it allowed him to say it would never happen while he sat at the Cabinet table.

With opinion poll ratings remaining low, Mr Clegg's task before 2015 is to demonstrate to voters that the LibDems are a party of competent government. That will be difficult if their strongest argument is that they have acted as a brake on more punitive Tory measures. So far, their main boast is that by raising the income tax threshold they have taken two million people out of taxation. That is laudable but it will not be enough, particularly if, as all the forecasts indicate, the economy is still struggling and LibDem-driven measures such as the Edinburgh-based Green Investment Bank are still in their infancy. Mr Clegg's claim that the longer you stand side by side with your opponents the easier it is to see the differences may be true and with Mr Cameron's backbenchers worrying about Ukip, Europe is a dividing line.

But after three years in coalition the most damaging fault line has been party-political jousting over reform of the House of Lords, with the Tories' failure to deliver resulting in the LibDems blocking boundary changes. If Mr Clegg is to recapture disenchanted voters, he must show there can be positive results to voting for his party.