Leading article: Cold winds are swirling around the third party

If confirmation were needed that politics is a harsh and unforgiving trade, yesterday's resignation of Sir Menzies Campbell as leader of the Liberal Democrats surely provides it. Despite Sir Menzies' reputation as a likeable and respected figure around Westminster, his party decided that his performance as leader was simply unsatisfactory. In the end, he was dispatched even more swiftly than his predecessor, Charles Kennedy.

There had been rumblings of discontent within the Liberal Democrats ever since Sir Menzies took over from Mr Kennedy last March. The expected boost in the party's electoral fortunes never materialised. There was a feeling from the very start that Sir Menzies lacked the charisma and zest to improve the party's fortunes and take its message to a wider political audience, a fear that was intensified by his early, bumbling performances at Prime Minister's Questions.

But what sealed Sir Menzies' fate in the end was simple electoral arithmetic. Labour's bounce in popularity after Gordon Brown became Prime Minister and the more recent fight-back by David Cameron's Conservative Party squeezed Liberal Democrats in the opinion polls. According to one poll, the party's popularity was down to 11 per cent, less than half its share of the vote in the 2005 general election. Many Liberal Democrat MPs – including some of their best and brightest talents – would probably have lost their seats if Mr Brown had called a snap election this autumn.

Nor did the fact that Mr Brown failed to fire the starting gun mean anything but a temporary stay of execution for Sir Menzies. The speculation had served to concentrate Liberal Democrat minds on their predicament. They could envisage no improvement in Sir Menzies' performance before 2009, when the election is now likely to be called. Indeed, if the Conservatives continue on their present upward trajectory, the squeeze on the Liberal Democrat is likely to become greater still.

It is a sad way for things to end for Sir Menzies. Had he become leader earlier in his career, things might have turned out very differently. He has been consistently impressive on the issue of Iraq, civil liberties and terrorism, where he has provided a statesmanlike voice of opposition to the Government. But with the departure of Mr Blair, Iraq inevitably declined in importance as a political issue. Sir Menzies never really came up with a compelling narrative to replace it. In the end he gave the impression of being a leader out of step with the frenetic world around him.

The Liberal Democrats were not short of political ideas. Under Sir Menzies' leadership they pioneered the ideas of using higher environmental taxes to offset cuts in other taxes – an approach that has since been adopted by the Conservatives and underlined how far ahead the party was on green issues. But Sir Menzies was unable to assert ownership of these ideas, compounding the frustration of those in his party.

Now thoughts inevitably turn to where the Liberal Democrats go from here. The party is blessed with a talented crop of politicians in Vince Cable, David Laws, Chris Huhne, and the most likely next leader, Nick Clegg. But whoever is chosen is unlikely to get a substantial fillip in the polls; the political conditions in which the third party of British politics finds itself are too unpromising for that. The new leader will be up against a pair of formidable political street fighters in Mr Cameron and Mr Brown, and he or she will struggle just as much as Sir Menzies to be heard amid the fray.

Yesterday was a wretched one in Sir Menzies' distinguished political career. But it was also, inescapably, a bleak day for the Liberal Democrats, which threw a harsh spotlight on the plight of the third party.