In a lengthy blogpost, the former Downing Street head of communications and strategy urged the party to choose “anyone but Corbyn”, despite having previously said he would not intervene in the contest.
He changed his mind about intervening because he believes the party would head for a “car crash, and more” under the Islington MP’s leadership.
“Whatever the niceness and the current warm glow, Corbyn will be a leader of the hard left, for the hard left, and espousing both general politics and specific positions that the public just are not going to accept in many of the seats that Labour is going to have to win to get back in power,” Campbell wrote.
In stark terms, he said that Labour’s consideration of Corbyn must stop if it wants to be a serious party of power rather than just a “party of protest that marches, campaigns, backs strikes, calls for ministerial resignations, more money for every cause going, shouts and bawls and fingerjabs”.
“Whilst I accept that I cannot survey the post-electoral scene and say with any certainty that a Labour party led by Andy Burnham, Yvette Cooper or Liz Kendall will win the next election, I think I can say with absolute certainty that a Corbyn-Tom Watson led Labour party will not.
“For that reason alone, I agree with Alan Johnson that what he called the madness of flirting with the idea of Corbyn as leader has to stop. That means no first preferences, no second preferences, no any preferences. It frankly means ABC: Anyone But Corbyn.”
Campbell also sought to remind Labour activists of occasions on which former leaders – Ed Miliband and Michael Foot – have convinced themselves that thousands of passionate supporters amount to the overall public support of the electorate.
He acknowledged that Corbyn is an “OK guy, a good MP, and his stance clearly chimes with many people’s views of anti-austerity”.
However, he warned: “Everything I have seen both of leadership, and of Labour, tells me Corbyn’s ability to lead and hold the party together is likely to be low; his ability to reach those parts of the country we have been losing, whether to the Tories, to Ukip or the SNP, will be even lower.”
Campbell ended with a call to Burnham, Cooper and Kendall to step up to the challenge and save the party by “showing that they too know how to make the weather in a campaign”.
Senior figures within the Labour establishment from the Blair and Brown era have been increasingly panicked in recent weeks about the prospect of a Corbyn victory.
Politicians including Alan Johnson have sounded the alarm about the electability of a leader from the left of the Labour party but the warnings do not appear to have dampened the momentum behind Corbyn’s campaign.
Others, including Labour MP Barry Sheerman, have called for a pause in the contest to stop “entryists” signing up to vote for Corbyn despite not being true party supporters.
However, Diane Abbott, who supported Corbyn’s nomination, said this was “ridiculous”.
She told the BBC’s World at One: “It would be absurd if you just halted an election because you were worried your side was going to lose. This election is being fought under rules that were agreed by the whole party last year. These people had the opportunity to object to them then. They’re only complaining because they’re worried they’ll lose.”
This article was written by Rowena Mason Political correspondent, for theguardian.com on Monday 10th August 2015 14.33 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010