Labour could be finished if Jeremy Corbyn wins the leadership, Alastair Campbell, Tony Blair’s former chief spin doctor, has said.

In a lengthy blogpost, the former Downing Street head of communications and strategy urges the party to choose “anyone but Corbyn”, despite having previously said he would not intervene in the contest.

He says he changed his mind about weighing in 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 writes.

In stark terms, he says 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,” he says, “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 seeks 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 acknowledges that Corbyn is an “OK guy, a good MP, and his stance clearly chimes with many people’s views of anti-austerity”.

However, he warns people who see Corbyn as “some kind of cross between Russell Brand, Nicola Sturgeon and their favourite uncle” should examine Labour’s recent history and the record of leaders from the left of the party.

“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,” he says, “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 ends the post 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 party but the warnings do not appear to have dampened the momentum behind Corbyn’s campaign.

Campbell’s comments came before a YouGov poll for the Times showed that Corbyn has the support of more than half of those with a vote in the Labour leadership contest.

The poll of 1,411 eligible voters found Corbyn had nearly doubled his lead over Andy Burnham in a week by 32 percentage points.

It gave him 53% - enough to win without a need to count second preferences - with Andy Burnham losing five points to 21%.

On Monday night, the Islington MP launched his policy for young people, who make up much of his growing support. Speaking in central London, Corbyn said he would restore the educational maintenance allowance for college students, allow housing benefit for under 21s, scrap university tuition fees, bring back student grants and introduce properly paid apprenticeship schemes as well as reducing the voting age to 16.

Meanwhile, Burnham gave an interview to the Huffington Post describing his frustration at having had to work as an unpaid intern for a local newspaper for three months while some of his Cambridge colleagues went straight on to jobs at national papers.

“Who says I’ve never done a real job? It was fairly real actually. It got me going, but I’ve always had this thing about unpaid internships because of it,” he said. In the interview, he also pledged to scrap the 5% rate of VAT on sanitary products, known as the “tampon tax”.

Amid growing consensus within Labour that Corbyn is heading for the most first preferences, his rivals are now trying to position themselves as the candidate most likely to beat him in a final round of voting.

Backers of Cooper say an analysis by independent psephologists suggest she has a 2% advantage compared with Burnham in her ability to get transfers.

Some within Labour, including Barry Sheerman, a Kendall supporter and MP, are even calling 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.”