Stella Creasy used the final Labour deputy leadership hustings to urge the party to focus on attacking the Conservatives rather than each other.
The Walthamstow MP was taking part in a Q&A hosted by The Huffington Post's Paul Waugh with rivals Tom Watson, Caroline Flint, Ben Bradshaw and Angela Eagle.
Creasy pointed to her decision to take on legal loan sharks as a backbench MP as an example of her campaigning abilities. "My best day as an MP was the day George Osborne wouldn’t be in the Today programme studio because we had made him, as a Labour movement, put a cap on the cost of credit," she said. "He couldn’t look me in the face."
And she said voters wanted Labour to try and make a difference despite being out of power. "I don’t mourn, I organise. And that gets me out of bed every single morning," she said.
"Let me tell you, I want to take it to George Osborne, I want to take it to David Cameron. Let's put them no notice they do not own the future of this country."
Creasy said: "Every single day we are looking internally Davis Cameron and George Osborne are off the hook."
Watson, who is considered the front runner to replace Harriet Harman in the deputy job, also said Labour could make a difference from Opposition - pointing to his campaign against phone hacking and child abuse as examples. "I think you can change from Opposition and I proved it from the backbenchers," he said.
And he joked that given the length of the deputy leadership campaign what had actually been getting him up in the morning was the thought of a "Premier Inn cooked breakfast" and "Shaun Keaveny on [BBC] Radio 6 music".
The candidates were also quizzed on how they would rebuild the party in the face of the threat from the SNP and Ukip. Eagle noted Labour only has 13,000 members in Scotland while 9,000 new members joined the party in the North West of England since the election alone.
She said the Scottish Labour party had got "too involved in internal machinations" rather than "reaching out" to voters. "We have to persuade people be they flirting with skip or having a full blown affair lets face wit with the SNP that we are on their side," she said.
Eagle also drew laughter when she mocked the "policy wonk" language used by politicians including phrases like "hard working people for work that's hard that really is working about hardworking people".