Labour former minister Frank Field has said Gordon Brown's first Queen's Speech as prime minister needs to shake the party out of its "despondency".
Ms Harman said the prime minister would provide strong leadership
He told GMTV Labour was in a "rut" and had yet to recover momentum after Mr Brown's decision not to call an early general election.
Mr Field said the PM must "look forward" to his vision for Britain.
Deputy Labour leader Harriet Harman said the party was unified and it was normal for a leader to be scrutinised.
Mr Brown has been urged several times to set out his vision for Britain, after his decision not to call an early election and amid improved poll ratings for the Tories.
Mr Field told GMTV: "Many of our side think that if [Lib Dem leadership candidate] Nick Clegg wins then that will actually take votes off Cameron because he looks like Cameron," he said.
"But if our despondency continues, and that's why this Queen's Speech is so important - that we get out of this rut - it might actually be Labour votes that Nick Clegg takes, that he's the sort of nice guy the Labour voting public might actually decide to back.
"If there's any flakiness in the vote, the flakiness might be on our side, which brings us back again to what our vision's about," he said.
He said the early election speculation had been designed to harm the Tories but had backfired and the government had to recover.
"It wasn't just shooting oneself in the foot; it was a double-blow - bullets into the head and one's actually got to try and recover from that," he said.
"That's in the past. We've actually got to look to what the government now wants to do and to change Britain."
Ms Harman told BBC Radio 4's World This Weekend programme it was normal for a party leader to be scrutinised but added: "There is an overall unity in the party because our underlying principles are very strong and very consistent."
"Gordon Brown does have a clear vision of the future based on the enduring principles and commitments he has always had. That is a commitment to backing up people who are working hard, making sure they can get on in their lives and ensuring that we have a fair and peaceful society."
Mr Field said the party was in a "rut"
She said he would be providing strong leadership but it was still "very early days".
Shortly after becoming prime minister, Mr Brown broke with tradition by giving a "pre-Queen's Speech" statement in July, at which he set out the programme he will be outlining on Tuesday.
Among legislation will be a Counter Terrorism Bill, which may seek to double the 28-day limit on holding terrorism suspects without charge, and a Health and Social Care Bill to establish a single regulator for the NHS and all adult social care providers.
Other measures will include raising the age at which children can leave education or training to 18 by 2015, as well as others covering children in care, pensions and constitutional reform.
But Ms Harman said reports the government's controversial ID cards scheme was to be watered down were "just speculation".