It was with lightning speed that Sir Menzies Campbell travelled the political journey from hero to zero.

Barely three weeks ago he stood, arms aloft, at the podium in Brighton taking the applause of his party conference, having turned the tables on his critics with a forceful speech laced with passion and even anger.

Last night he quit so abruptly after his brief tenure as leader of the Liberal Democrats that he did not stay to make his resignation statement, let alone until the outcome of the leadership election that will now follow.

Even by the tumultuous standards of the past few weeks, during which David Cameron was written off but fought back with a vengeance, and Gordon Brown commanded double-digit poll leads before shooting himself in the foot by flirting with an early election, Sir Menzies’ sudden demise was extraordinary.

Having declared repeatedly since taking charge that he would lead his party into the next election and beyond, Sir Menzies suddenly became the greatest casualty of the autumn general election that never happened.

The cheers that greeted his forceful — and final — party conference speech were genuine as he vowed that the Liberal Democrats would break the “cosy consensus” that existed between Labour and the Conservatives on a string of policy issues.

But Lib Dem members gathered in Brighton were braced for the prospect of an imminent election. Despite skirmishing between Nick Clegg and Chris Huhne, the rivals for his crown, activists knew that unity would be vital to any campaign.

Gordon Brown’s belated announcement that there would be no poll this year, and that it was “very unlikely” there would be in 2008, suddenly presented the Liberal Democrats with an 18-month window during which they could rescue their position.

For, despite protests to the contrary from Lib Dem MPs, their plummeting poll ratings were behind the sense of deepening alarm. The most recent Times/Populus poll found the Lib Dems on 12 per cent, the lowest figure recorded by Populus; three other polls put them still lower on 11 per cent.

More ominously for Sir Menzies, 66, his personal ratings on leadership qualities were consistently poor, worse than those of his predecessor, Charles Kennedy. After a difficult start, as he struggled in the Commons first as acting leader after Mr Kennedy’s resignation and faced continuing criticism on his election by party members, Sir Menzies never fully recovered; the public, which sees relatively little of Lib Dem leaders, had made up its mind.

As Lib Dem MPs returned to the Commons after the party conference season, conscious that their support was crumbling amid a squeeze from Mr Brown’s “reaching out” strategy and Mr Cameron’s cunning pitch as a “liberal Conservative”, Sir Menzies tried a pre-emptive strike.

He gave a series of robust interviews, declaring that he remained determined to stick to his timetable of staying until the next election. He confided to colleagues that May 2009 had long been marked in his diary as his resignation date and told them nothing had changed.

Some friends of Mr Clegg also thought his performance in Brighton, when he undermined Sir Menzies by admitting that he would probably stand when the leader quit, showed he was not yet ready and needed more time; Mr Huhne was thought to have more to gain from an earlier contest having raised his profile by standing in last year’s leadership election.

Allies of Mr Huhne were being accused last night of a brutal briefing campaign to destabilise Sir Menzies and hasten a leadership contest before Mr Clegg’s stature and support base within the party grew.

Lib Dem MPs, the majority of whom backed Sir Menzies to be leader last year, discussed their plight but were reluctant to confront him. For one thing, he had proved himself a tough leader and at times of dissent often challenged critics to tell him their views in person. He left little doubt that they faced dismissal from the front bench if they did.

In what became his last speech as leader, Sir Menzies told a regional conference in Suffolk at the weekend that he answered to them and not to the media. His decision to fall on his sword, rather than lead his party in the face of crumbling support and negative headlines, suggests otherwise.

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