Steve Richards: Why does Sir Menzies look morose? He is lucky to be leader at a time of such volatility

There is a growing expectation that there will be a hung parliament after the next election

What is it about being the leader of the Liberal Democrats? Since winning the party's leadership contest in January, Sir Menzies Campbell has looked awkward and ill at ease in his public appearances, as if he had been the defeated candidate rather than the triumphant victor. Before he got the post, he was the most authoritative figure in his party and displayed a zeal for politics. Now he has won he struggles to assert his authority, and seems to be miserable.

He follows a familiar pattern. Charles Kennedy was a self-confident and exuberant political performer before he became leader. After he acquired the crown, Mr Kennedy became nervously self-effacing. Paddy Ashdown never looked as if he was enjoying himself very much. As leader of the Liberals, David Steel was forced to take a break from the job, suffering from exhaustion. Under much more pressure, and playing for higher stakes, Messrs Blair and Cameron look as if they are having a ball quite a lot of the time. Leaders of the smaller, third political party behave as if they are burdened by the demands of high office when they are far away from power.

Perhaps the distance from government is part of the reason for the apparent gloom. The Conservatives seek power in a single leap. At least their leaders can contemplate the prospect of a stint in Downing Street. The Liberal Democrats hope at each election to form a bigger parliamentary party, not quite the same adrenalin-inducing aspiration. Probably also the weekly session of Prime Minister's Question Time becomes a demoralising experience. No Liberal leader has been any good at it. Even Roy Jenkins, a confident parliamentary performer on Labour's front bench, found it impossible to make his mark in the Commons as leader of the SDP. Judgements on Sir Menzies' stuttering start are based largely on his nervous performances at Prime Minister's Question Time.

What a misleading contrast: David Cameron's effortlessly charming and energetic public performances create an impression that since last autumn the Conservatives have changed beyond recognition. Similarly, Sir Menzies' lacklustre public interventions suggest the Liberal Democrats have staggered along, hardly changing at all.

The opposite is the case. In policy terms, the Conservatives have made no significant changes. Still they struggle with the consequences of their extreme Euroscepticism, support the war against Iraq, and promise, as if by magic, to improve public services while cutting taxes. We await the policies that match the tone of Mr Cameron's speeches, in which he implies progressive support for a more active state. If we get the policies, how will his party respond? The big test for Mr Cameron is the outcome of his party's policy review in a year's time, one of the pivotal moments in this parliament.

Meanwhile, Sir Menzies moves with the speed of an Olympic runner. In a wide-ranging speech today, he will announce sweeping changes to his party's economic policies. In particular, he will propose big reductions in income tax for those on middle and lower incomes, and confirm that he no longer supports a new top rate of tax for high earners. The Liberal Democrats will find other ways to redistribute, mainly through changes to capital gains tax. In theory, the reductions in income tax would be paid for by a range of green taxes, including substantial increases in tax for owners of big cars. Sir Menzies may have been the new quiet man of British politics. Today he launches a noisy revolution in his party's approach to taxation.

Unsurprisingly, the revolution is flawed. If people become greener to avoid the new proposed taxes, the Government would lose billions. Alternatively, if people continue with their environmentally unfriendly habits and pay their green taxes, there would be no improvements to the environment. Either the party's sums will not add up or the revolution will have no impact on the environment.

Still, the Liberal Democrats did not get where they are today by making their sums add up. In the rigid, mendacious debates over economic policy, they play a different and important role. At their most agile, they adopt policies that symbolically are one step ahead of the other parties. In the 1990s, they argued alone the case for higher taxes to pay for essential increases in public spending. It was largely irrelevant that their specific proposal - a 1p increase on the basic rate of income tax - would have made little difference. Their function was to make the case for higher spending, a stance vindicated by the new consensus between the parties on the shameful underinvestment in public services before 1997.

Now they leap ahead once more. In making their radical proposals for green taxes, they pose a challenge to the other parties that affect an environmental concern. In particular, how will the Conservatives respond to this dual challenge of income tax cuts and environmentally friendly proposals? Politically, the new policies are astutely judged.

Sir Menzies unveils his revolution in a political context that is also more fruitful than it seems. There is a growing expectation across the political spectrum that there will be a hung parliament after the next election. Such an outcome would place the Liberal Democrats in a powerful bargaining position. They could lose many seats and yet be more powerful than they are now.

I should add that I do not believe there will be a hung parliament next time. There has been only one since the Second World War, in the unusual circumstances of February 1974. There is no reason why it will happen again under the current voting system. Nor is Sir Menzies dewy-eyed about a hung parliament. He knows that such an outcome would quite possibly lead to a political and economic crisis for the country, rather than a new paradise for his party.

But precisely what will happen in the future is irrelevant for the time being. The fact that so many commentators and politicians contemplate the possibility gives the Liberal Democrats relevance now. Mr Cameron has asked Sir Menzies to meet him to seek out common ground. Sir Menzies has agreed to a meeting. Gordon Brown and Sir Menzies are old friends. They keep in touch. Some Brownites express the hope that the Liberal Democrats will become part of their so-called progressive consensus. Sir Menzies is in the odd position of being widely and prematurely dismissed in the media as a doomed leader, while being wooed by the two men who would like to be Prime Minister after the next election. He is lucky to be leading a party at a time of unusual political volatility.

Unless, of course, the volatility sweeps him up. Sir Menzies must break the tradition established by his predecessors and look as if he enjoys being leader of his party. If he fails to do so, very quickly the narrative will remain entirely about the weaknesses of the messenger, and the boldly innovative message will be lost.

s.richards@independent.co.uk