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Owen Smith has said Jeremy Corbyn's political principles are "just hot air" because there is no real prospect of him leading the Labour Party into power.
The MP, who is standing for the leadership, said that Mr Corbyn was unelectable so there was "no point" to the Labour party at present.
He said the party needed to "get up off its knees" and start looking like a "credible" opposition.
"We all want the Labour Party to be a powerful opposition to the Tories, we all want us to be a united opposition, but we haven't been in recent months," he told Sky News.
"He's a principled man and someone who has got deep Labour values, but our question is 'can he take the Labour party to where we need to be?'
"Because without winning elections, without Labour being a serious party of government again, then all the principles are just hot air."
He added: "What's the point of being in this game?"
Mr Smith has laid out his leadership stall with the promise of a £200bn investment in skills and employment and has also said he would have equal representation of men and women in his shadow cabinet.
Meanwhile Mr Corbyn has laid out his commitment to tackle the "five ills" of 21st century Britain: inequality, neglect, insecurity, prejudice and discrimination.
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Mr Smith's words follow claims by former shadow cabinet minister Seema Malhotra, who opposes Mr Corbyn's leadership, that a close aide of his "illegally" entered her office following a campaign of "intimidation" against her.
Mr Corbyn's team has said the aide was an office manager who needs to prepare Ms Malhotra's old office for a new staff member following her resignation.
Meanwhile, the latest poll shows Mr Corbyn has twice the support of Mr Smith among Labour party members and is set to win the leadership contest in September this year.
Some 54 per cent back Mr Corbyn to remain as leader, while just 22 per cent say they will vote for Mr Smith, according to the Opinim/Observer poll.
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