Andy Burnham and Yvette Cooper have been branded “continuity Miliband” by a leading supporter of Liz Kendall, in a sign that the Labour leadership contest is turning sour.

John Woodcock, the Barrow and Furness MP and a leading figure in the Kendall campaign, said he was prompted to speak out by two “heart-sinking” moments in a Labour leadership hustings on Sunday.

In a blog, he said Burnham and Cooper – the two favourites to succeed Ed Miliband – were “offering only superficial modifications to the Miliband approach that just consigned us to one of the worst defeats in our history”.

Apart from one newspaper story quoting an unnamed source dismissing Kendall as representative of “Taliban New Labour”, the tone of the contest has until now been positive. Woodcock said he was speaking out because he was frustrated by Burnham and Cooper’s reluctance to accept the need for change, but the Kendall campaign’s decision to go negative may also be triggered by the perception that, after a promising start, she is now trailing behind her two more experienced rivals.

Woodcock said that, although all candidates professed to be committed to change, Kendall was the only candidate serious about restoring Labour’s economic credibility.

“If those who seek to take his place think the route to victory in the leadership contest is continuity Miliband with a different accent or gender, or with a higher level of emotional connection, they will consign Labour to another defeat at the next general election,” Woodcock said in blog seen by the Guardian and due to be published on Monday.

He said the two “heart-sinking” moments at the hustings were Burnham saying that he would give councils the power to compulsory purchase properties from slum landlords – a move Woodcock described as “precisely the kind of measure the public rejected at the ballot box as unworkable” – and Cooper not being willing to identify anything Miliband got wrong during his leadership.

A source in the Cooper camp said Woodcock’s article was “pretty desperate stuff” and that Cooper had come up with new ideas, not least about how Labour needed to improve its relationship with business.

A separate source, from Burnham’s team, said he was the candidate who could achieve “big change” by attracting voters from all over the UK, and that Kendall had not even managed to secure the support of any of the shadow health ministers who have worked with her over the last five years.

In his article, Woodcock also acknowledged that Jeremy Corbyn, the fourth leadership candidate who is seen as a rank outsider because of his leftwing views, was getting a “great” reception at leadership hustings.

But Woodcock claimed this did not indicate likely success in the contest.

“A round of applause in a hall of activists does not necessarily translate to a majority of votes in the wider selectorate of members and registered supporters. If it does then we can wave goodbye to any hope of electability for the foreseeable future,” Woodcock said.