Yvette Cooper has warned Labour will lose the next general election if it tries to "swerve all over the place" in a thinly-veiled attack on her leadership rival Andy Burnham.
The Shadow Home Secretary's comment came as Mr Burnham attempted to woo supporters of frontrunner Jeremy Corbyn, days after criticising his policies as lacking "credibility.
Meanwhile, the Daily Telegraph reported Lord Mandelson tried to halt Mr Corbyn’s march in the Labour leadership race by attempting to persuade the other candidates to pull out.
The Labour grandee allegedly presented a plan to Ms Cooper, Mr Burnham and Liz Kendall asking them to quit to bring the contest to a halt last week prior to the release of the ballot papers.
Ms Cooper, who re-energised her campaign last week after a speech criticising Mr Corbyn for offering "old solutions", today took a swipe at Mr Burnham as the two fight to be the favoured "Anyone But Corbyn" candidate.
In an interview on the Today programme this morning, she said: "We can't move our narrow party a bit to the left or a bit to the right. But neither can we swerve all over the place - because we will crash and we will lose in 2020. And then we let people down."
Mr Burnham will today say he is the only party that can "unite" the party, adding: "Jeremy has brought to this race. I want to capture that and involve Jeremy and his team in rebuilding our party from the bottom up.
"I want to show how I can take the best ideas of the other candidates, where there is common ground between us, and use them to shape my radical vision.
"In that way, we will come out of this contest as a strong and united team ready to take the fight to the Tories."