Leading article: This week has shown that the election is far from over

Gordon Brown's fight-back demands a clear response from David Cameron

Suddenly the next general election seems much closer and the outline of the battle more clearly defined. This weekend marks the mid-point of perhaps the most significant fortnight in British politics since Gordon Brown bottled an early election two years ago. Next week the Chancellor, Alistair Darling, delivers his pre-Budget report and Gordon Brown attends the Copenhagen Summit on climate change. Surprisingly, though, the events of the last few days may prove to be at least as significant in shaping the divide between the parties.

Nick Clegg unveiled his party's bold tax proposals. A poll in The Independent suggested a hung Parliament was a distinct possibility. The next day Gordon Brown and David Cameron clashed in the liveliest Prime Minister's Questions since Mr Brown became Prime Minister. In between, both leaders delivered major speeches: Mr Brown on Afghanistan; Mr Cameron on excessive regulation.

The accumulated impact of this sequence is changing the dynamics of British politics. The change is not yet dramatic. The polls still show a substantial Conservative lead, and the Labour government remains deeply unpopular. Nonetheless, Mr Brown is showing signs of a striking fight-back in ways that raise profound questions about Mr Cameron and the party he leads.

Mr Brown will not get very far by reviving so-called "class warfare"; it also smacks of hypocrisy for a New Labour politician. But the point he raised on Wednesday about the candidacy of Zac Goldsmith and his non-domicile status was entirely valid. Does Mr Cameron believe a party that aspires to govern in difficult economic times should be represented by wealthy candidates who have done their best to avoid paying tax?

The contrast between Mr Brown's major speech this week and Mr Cameron's chosen theme was also marked. While Mr Brown outlined a strategy for Afghanistan, one that is flawed but could command support in the run-up to the election, Mr Cameron highlighted some poorly researched clichés about the impact on daily life of health and safety regulations. Indeed, Mr Cameron seems to have gone out of his way to please his new allies in the right-wing press with the development of a crude anti-state message, backed by glib assumptions that do not stand up to scrutiny. This is a foolish and wrong-headed strategy to pursue.

There was much speculation this week about whether Mr Brown is pursuing a "core vote" strategy. But the same question could be asked of Mr Cameron. He must decide what sort of leader he wants to be: one who speaks with confidence to a wider audience, as in his early phase as leader, or one who retreats into following in the footsteps of his recent predecessors.

Next week will be a big test for the Government, with both the Pre-Budget Report and the Copenhagen Summit. But it will be a defining moment too for Mr Cameron. Will he take on his party's army of climate change sceptics with specific green policies and support any attempts to get an agreement that will involve some tough policy measures in the UK? Will his and George Osborne's response to the Pre-Budget Report be more rounded than a shrill call for immediate cuts? Are they capable of coming up with tax policies as fair and as radical as those unveiled by the Liberal Democrats?

If Mr Cameron wants voters to believe that his party has changed, he needs to resolve these issues. He has left it late, and if he gets them wrong, the next election is far from over.