Lisa Nandy has accused her rivals for the Labour leadership of putting the party’s survival at risk by refusing to have an “honest reckoning” about its devastating general election defeat.

In an outspoken interview with the Observer, in the week that ballot papers were sent out to more than 500,000 Labour members, registered supporters and affiliates, Nandy said she felt she was the only candidate who had been honest about Labour’s failings and the challenges it now faces.

Accusing her remaining opponents – Keir Starmer and Rebecca Long-Bailey – of failing to face the truth, she said: “It has been frustrating to hear a sort of narrative develop that says that we broadly got things right on Brexit, broadly got things right on policy, and broadly got things right on leadership. So how could it possibly be that we have ended up with our worst election defeat for a hundred years?”

The MP for Wigan, who has impressed audiences around the country, said millions of people whose families had had Labour “in their blood” for generations had deserted it at the election – yet her rivals seemed not to understand that there was now an existential risk for their party. “These are people who voted Labour for 100 years, so this was no ordinary break. And we have got up to face up to that – and I do not believe at this stage that there is anybody else in the contest who is having an honest reckoning with that,” she said.

“People are looking to us to see whether we’ve understood it and that we’ve got it. But there is a very small window and that window will close. And if we don’t get this right now, there will be no Labour party to vote for in four years’ time.

“I came into this contest to tell the truth about what has happened to the party and to the country in order that we might come out the other side on 4 April with a leader who is capable of winning in four years’ time. We can win in four years’ time, but we can only do it if we tell the truth.”

Her remarks follow several weeks of hustings and meetings around the country during which the candidates appeared to have signed a non-aggression pact in an attempt to prevent the events becoming blame games that would further harm Labour’s standing among voters.

According to a survey by YouGov, Starmer, the shadow Brexit secretary, is on course to win 53% of members’ first-preference votes while Long-Bailey, the candidate most closely associated with Corbyn, will emerge with 31%. According to YouGov, Nandy would take 16% of first-preference votes. Her supporters insist, however, that she is still very much in the race and could attract large numbers of second-preference votes.

Nandy says she has been far more critical of the party over its stance on Brexit and antisemitism than her rivals were, while criticising the party’s election manifesto as a wish list that no one in Labour communities believed was realistic or affordable. She also sought to differentiate herself by stressing the need for Labour to become far more “rooted in communities” so it can prove to people what it can deliver for them locally.

Nandy revealed that she would give women in the party the power to rule on sexual-harassment cases, some of which remain unresolved after several years of Corbyn’s leadership. “I think the strong suspicion, in relation to sexual harassment and antisemitism, is that there is one rule for some people in the party and another rule for others.

“And that is why an independent complaints process is so important. Because you cannot believe that just by changing the leadership, that somehow you get away from this notion of factional or personal interference in the process. So it’s really important to take this out of the leadership and away from any political considerations, any personal considerations.

“There cannot be one rule for friends of the leader and another rule for others. So the first thing you do is you get women together in order to determine what constitutes harassment and unacceptable behaviour. Second thing you have to do is set up an independent complaints process.

“Thirdly, you have to have a very strong tough, robust whistleblower policy within the Labour party. We saw it over antisemitism, where people who came forward were then targeted, but the same is true of women who have spoken out against antisocial [behaviour], against sexual harassment as well.”