The resignation of Sir Menzies Campbell as leader of the Liberal Democrats brings a sharp reality check to British politics after the most volatile month in recent times. The gravitas and analytical ability Sir Ming brought from his days as a QC served him well in political debate and his expertise on foreign affairs commanded cross-party respect. Routinely described as urbane and suave, his statesmanlike demeanour bafflingly failed to translate into convincing authority as LibDem leader.

The resignation of Sir Menzies Campbell as leader of the Liberal Democrats brings a sharp reality check to British politics after the most volatile month in recent times. The gravitas and analytical ability Sir Ming brought from his days as a QC served him well in political debate and his expertise on foreign affairs commanded cross-party respect. Routinely described as urbane and suave, his statesmanlike demeanour bafflingly failed to translate into convincing authority as LibDem leader.

His reputation was founded on forensically-researched and devastatingly-delivered challenges on foreign policy, particularly over the Iraq war, in the House of Commons. There was a time - just a few years ago - when Charles Kennedy was at the height of his charismatic powers and Ming Campbell was the clever, careful, dependable voice of good sense, so much so that the party matched the mood of the country and the poll ratings made them a genuine third force. When his problems with alcohol made Mr Kennedy's position untenable, it was Sir Ming's sure hand on the tiller as deputy leader which saw his solid track record win the day.

As the figurehead, however, he failed to live up to the promise of his incisive, articulate performances in the Commons. It was as if, like the war in Iraq itself, the rapier response with which he questioned the premise had been replaced by the daily reality and his only weapon was a plodding determination. Even a former Olympic sprinter can look tired beside younger challengers, and his critics made the most of it. Against a distrusted Tony Blair and a failing Iain Duncan Smith, Sir Ming cut the mustard; against a newly-green David Cameron and newly-installed Gordon Brown, his chief credentials - progressive environmental policies and palpable principle - were no longer unique. Worse, however, was the failure to claim full credit for taking the initiative on taxes and the environment. That should be a key lesson for his successor, who will certainly gain from a new, relatively youthful image. Sir Menzies's instinct is thoroughly Liberal and, with the centre ground more hotly-contested than ever, the direction taken by a new LibDem leader will determine much of the political battlegound in the run-up to the next election.

His conference speech in Brighton failed to rally the troops sufficiently to see off the persistent sniping about his age. The even-tempered rebuttals, and explanations that experience is valuable, sounded sadly unconvincing when not matched by his less than charismatic performances. Although he spoke in recent days of leading the party "with ambition", that was always for policies rather than personal glory. Against the hungrier ambitions of the other party leaders, it made him seem an unelectable alternative. Urged to go with dignity, he did so decisively and in characteristically gentlemanly fashion. He took on the leadership for the sake of providing stability in the party and the timing of his resignation is equally focused on the greater good, providing his successor with time to rebuild before the next election. He was undone by his own best qualities and the loss of a thinking, courteous, principled leader should be a matter of real regret.