The leadership candidate said the party had been losing votes in its heartlands for a "long time" and had lost four general elections in a row.
People wanted "fundamental change" but did not trust Labour to deliver it, he told the BBC's Laura Kuenssberg.
He vowed to restore trust in Labour "as a force for good and a force for change" and end factional infighting.
But he refused to say whether his politics were closer to Tony Blair or Jeremy Corbyn, saying: "I want to lead a Labour Party that is trusted enough to bring about fundamental change.
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