Voters change governments for two basic reasons: to rid themselves of an old one that is failing, or to usher in a new one that offers important and inspiring change. With nearly two years left before Gordon Brown must call an election, dismay with his administration seems ever more entrenched. The Conservatives are riding high and quietly preparing to take power. But it is not at all clear what they would do with it or whether, truly, they deserve it.

Polls giving David Cameron a lead of up to 20 points have put a spring in his step. But as measures of his ability to sustain the Tories' political dominance until 2010, they are fragile. As indicators of whether his party represents a credible alternative to Labour, they are meaningless.

The Tory leadership is said to have a morbid fear of complacency. And yet there is a self- assurance about the Conservatives these days that has a whiff of smugness. Their success is not rooted in popular enthusiasm for the Tory project, but the public's weariness with Mr Brown. This is by no means a mandate for government.

The Conservatives are equally fearful of repeating Tony Blair's mistake of entering Downing Street unprepared for government. So they have set up an “implementation unit”that is coaching frontbenchers on how to rush proposals for new schools and welfare reform into law.

There is no shortage of detail in these proposals. But there is a serious shortage of inspiration holding them together. It is easy to see what the Conservatives are against. Their critique of Mr Brown's Governmment chimes with the current mood. But what are they for? Some Tory frontbenchers say that the Conservatives promise “competent, sensible government”. But is that all? Is a pledge not to lose personal data down the back of the sofa at Revenue & Customs or leave classified documents on the train home all we want from government?

David Davis wants the Tory party to be about civil liberties. His colleagues can barely hide their disdain for him. Mr Cameron has promised to do the unimaginable by reversing social decline in much the same way that Margaret Thatcher disproved the sceptics and reversed economic decline, but the question is how?

Mr Cameron has let an assumption take hold that it will be possible not just to win, but to govern, without a unifying theme. His policy review has produced ideas in abundance, but little sense of direction. One result is that Tory “green papers” such as last month's on healthcare reform fail to frame national debate as they should with a government in such disarray.

Mr Cameron's skill as a communicator and the Conservative Party's justifiable focus on tactical management of the media only exacerbate the problem. Precisely because “Dave” looks good on camera, he can seem stylish but insubstantial. This week, he chose to mark the 60th anniversary of the NHS by hosting a live hospital gig for the stars of Britain's Got Talent. It did little to help to assuage the fears that he and his party have yet to address the difficult issues of public policy facing the next government.

It is unclear as yet what real choices the Conservatives will offer voters on tax, healthcare and the environment, or whether Mr Cameron can galvanise Tory activists to make his rebranding of the party more than that, to the point that it represents Middle England as new Labour did by 1997. He has not done so yet.

His challenge is to offer British voters a real choice. The Tory modernisers have time - too much, they may think. But they also have more work to do than they imagine. No one wants a government by default. It is not enough for them to preserve their poll lead and win in 2010. They have yet to prove that they are worthy of it.