Steve Richards: Right from the start, there was a sense that Sir Ming's moment had come too late

In going now, the LibDem leader has done his party a favour. But his successor faces a daunting prospect

Leaders become vulnerable in periods of wildly oscillating political moods. Three weeks ago there was some speculation that David Cameron would not survive for much longer. Now he enjoys a second honeymoon. Gordon Brown walked on water in September. Suddenly he struggles to stay afloat. But amid the extraordinary turbulence it is Sir Menzies Campbell who becomes the first victim, a leader who found he had no choice but to resign.

Nobody envisaged events would move quite as fast as this. Sir Ming finished his party conference last month on something of a high, delivering a well received speech and contemplating a possible early election campaign in which he would have been the leader. Now he has announced his resignation.

In going now, Sir Ming has done his party a favour. If he had not acted voluntarily, his troubled leadership would have remained the only big issue associated with the Liberal Democrats. The public criticism would have intensified and fed on itself. As it turned out, a few critical comments over the last few days have been enough to bring about a sudden resignation from a leader who was declaring genuinely at the weekend his determination to stay on.

His colleague, Simon Hughes, told me on GMTV's Sunday Programme that Sir Ming had to improve his performance. That comment was enough in itself to provoke newspaper reports of a leadership crisis. Yesterday, other senior figures spoke out more critically in public. Even the ultra-loyal deputy, Vince Cable, admitted on the BBC that Sir Ming's leadership was under discussion. A few hours later the leader was gone.

Over the last few weeks three new factors fuelled what was already a fairly febrile mood within the Lib Dems: David Cameron's revival, Gordon Brown's decision to postpone an election probably until 2009, and the polls that showed a further decline in support for the third party.

In this even gloomier context Sir Ming acted now rather than stubbornly stay on and resist the onslaught. He knows what that can be like from the other side, as he became increasingly frustrated at Charles Kennedy's determination to remain as leader until the parliamentary coup in January of last year, an event in which Sir Ming played a still ill-defined role. After Kennedy finally resigned, he acquired the throne, but from the beginning of his leadership and those first nervous performances at Prime Minister's Questions, there was a sense that his moment had come too late. As the party's foreign affairs spokesman, Sir Ming punched above his weight. As a leader he became a strangely diminished figure. For some political figures leadership enhances their authority, but it does not always do so.

Potentially his departure changes politics in two profound ways. The Conservatives' sudden surge in popularity has been largely at the expense of the Liberal Democrats. A popular successor might claw that support back at the expense of the Conservatives. David Cameron will be a little uneasy at the news of Sir Ming's resignation. But Gordon Brown will be reflecting on the implications too and will not be unequivocally joyful. Brown and Sir Ming got on well. Unlike Kennedy, Sir Ming admitted openly that he was on the left of centre. In the event of a hung parliament his instincts would have been to work with Labour. Perhaps a successor will think differently.

But I would be wary of taking these hypotheses too far. They suggest that the Liberal Democrats call the tunes to which other parties dance. Most of the time the opposite is the case. The battle for the centre ground between the two bigger parties means there is much less room for the third party. As long as Cameron clings to that space, the Liberal Democrats will struggle.

If Cameron gives up the ground they will breathe more freely. There are limits to what any leader of a smaller party can do in these circumstances. As for a hung parliament, the dynamics in such circumstances will be determined by the electoral arithmetic at the time and not by the rapport of the leaders involved.

Still, for the potential candidates in the Liberal Democrats, this is a big moment that has arrived earlier than any of them had imagined. At their conference last month, The Independent staged a debate between Nick Clegg and Chris Huhne. Even the two participants joked that it felt like a leadership hustings. No doubt they will both stand, with Clegg starting out as the favourite.

Clegg has an engaging charm, is good with the media and has brought an innovative approach to his home affairs brief. Huhne is a formidable economist and has a mischievous humour that surfaces more in private than on the conference platform or in the media. Huhne has put together some impressive green policies and presents them with an economist's credibility. Both are more than credible candidates, but the political context for whoever wins remains daunting.

As a leader, Sir Ming stabilised the Liberal Democrats after a darkly traumatic period. He made several internal changes that impressed his parliamentary colleagues. But in spite of announcing a range of distinctive policies, he never gave the third party a clear national role. Paddy Ashdown managed to do so by moving close to Tony Blair. Charles Kennedy made a distinctive pitch because of his opposition to the war against Iraq. Sir Ming has not had an issue or a strategic opening to make similar waves.

Without an issue or an opening there is a danger always that the Liberal Democrats seem irrelevant on the national stage. In their excitement the potential candidates should not forget this. Parties receive media attention during leadership contests. The interest can fade when the contest is over.

But as a result of Sir Ming's sudden decision, at least a successor will have some time to make headway. He will inherit a better working party machine and some bold and detailed policies. Still he must discover a distinctive purpose in the new political context shaped at a national level by Brown and Cameron. For the Lib Dems, the stakes could not be higher. They have got rid of one leader and lost another in the space of less than two years. Like the Conservatives after 1997, a series of leadership contests can start to look silly at best. Tory MPs used to enjoy the leadership contests only to find that eternal opposition loomed once the fun as over.

The next leader cannot afford to fail, although definition of success and failure for the Lib Dems at a national level is not clear. Is it winning a few more seats and remaining with the purity of opposition? Is it securing a hung parliament and forming a coalition? If so, which party would it be able to work more closely with? The ambiguity is not resolved by the sudden departure of their leader, a decent man who will be feeling bewildered at what has happened in recent weeks.

s.richards@independent.co.uk