For Charles Kennedy’s many critics in his own party, the need for a change was not simply driven by their former leader’s drink problem. The arrival of David Cameron as Tory leader also forced the point home.

Third parties are obsessed with opinion polls. A good poll showing is an antidote to an electoral system they see as outrageously biased against them. A bad showing, in contrast, offers only the prospect of a steady drift into distant obscurity.

Before the May election, Kennedy boasted that the Liberal Democrats could replace the Tories as the main opposition. Since then that boast has looked increasingly hollow.

At the election the Lib Dems secured 23% of the vote, compared with 33% for the Tories and 36% for Labour. That was the party’s high-water mark.

Kennedy’s failure to build on a good election showing, with an absence of new initiatives and a lack of political visibility, resulted in an early loss of support. While the Lib Dems dithered they lost ground — initially to Labour. According to YouGov, the online pollsters, Labour’s support rose to between 38% and 40% over the summer while the Lib Dems slipped to 20%.

Kennedy, in his party conference speech in September, was not too worried. He predicted that “as the Blair era comes to an end and the prime minister desperately tries to cement his legacy, the Labour party will begin to turn in on itself”. As for the Tories, they were “simply not up to the job; they are too inward looking — too divided”. He was wrong; the Tories were about to get a new lease of life, and it would be partly at his expense.

Lib Dem MPs deny that the move to get rid of Kennedy was a knee-jerk reaction to Cameron’s rise. And when Kennedy resigned yesterday, he appealed to his party not to get “distracted by the machinations” of other parties. But the Cameron effect has been undeniable.

The new Tory leader has given his party a boost at the expense of both Labour and the Lib Dems. On YouGov figures, Cameron pushed the Tories to 37%, almost twice the level of backing for the Lib Dems, down to 18%.

More significantly, Kennedy’s personal leadership took a battering in the polls. A year ago he had a hugely positive approval rating, with nearly three times as many people saying he was doing well as badly — 61% to 23%, a 38-point positive rating. He held on to this position at the May election.

Last month, however, when asked the same question, only 45% of voters said that Kennedy was doing a good job, while 37% said he was doing badly, a net eight-point positive rating. Even before his public admission of his drink problem, voters sensed the Lib Dem leader was sinking.

Kennedy’s main electoral asset, that he was a likeable sort of chap, was useful when up against an unpopular Tony Blair and “nasty” Tory leaders such as Iain Duncan Smith and Michael Howard. But faced with a new Tory leader who is young, likeable and making a convincing effort to show that he is in touch with the concerns of ordinary people, Kennedy’s personality-based appeal quickly began to wear thin.

When compared with Cameron’s articulateness and ready grasp of policy the Lib Dem leader looked, in contrast, like a man out of touch and out of place Add in the drink factor and Kennedy, to his own MPs, began to look like a bumbling amateur.

“Perhaps we could have put up with Charles’s drinking if he was a Winston Churchill when sober,” said one Lib Dem MP. “But he wasn’t and that was becoming increasingly clear.”

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