Leading article: Beware the siren voices of alliance

No one can accuse David Cameron of lacking chutzpah. It was a cheeky piece of opportunism for the Conservative leader to call on the Liberal Democrats and the Green Party to join the Conservatives in, as he put it, "creating a new progressive alliance to decentralise British politics".

But no one should blame the Liberal Democrats either for their sharp rebuff. If Mr Cameron had hoped to catch them in a moment of weakness as their acting leader, Vince Cable, prepares to give way to the new leader, when the result of the contest is announced tomorrow he overstepped the mark. In Mr Cable, and the former leader, Charles Kennedy, the Liberal Democrats had vigilant elder statesmen ready and willing to defend their party's interests.

The Liberal Democrats cannot afford to enter any alliance formal or informal at this stage. Nor do they need to. Barring accidents, a general election is at least two years away. The new leader, whether it be Nick Clegg or Chris Huhne, has time to consolidate his authority and stake out new territory. Why begin by dancing to Mr Cameron's tune?

In saying he wants to put pressure on the Government to decentralise power, the Conservative leader has chosen an issue that is close to Liberal Democrat hearts. And it may be that this is an issue on which the two parties can usefully make common cause. But there are different ways to decentralise government. The new Liberal Democrat leader needs to begin by charting his own course.

The timing of Mr Cameron's overture, however speculative, though, is useful. It is a warning to the Liberal Democrats, once they emerge from their latest bout of introspection, that Mr Cameron is still interested in stealing their political clothes, especially those of a localist and greenish hue. They must be on the look-out for chances to broaden their appeal, while being careful not to dilute the qualities that make them distinctive.

But Mr Cameron's intervention should remind the new Liberal Democrat leader, too, that the next election could give the party its best shot at sharing power for many years. He must always bear that in mind and avoid all appearance of third-party amateurishness. This is a time when the appearance of substance and authority could make all the difference.

To lean too far towards one major party or other would also be a big mistake, however seductive the calls from Cameron-land might sound. It would be folly for any Liberal Democrat leader with a realistic shot at a slice of power, to surrender any of his cards before the bargaining is for real.