Nick Clegg has hinted at a possible post-election deal with Labour, in a move that will be seen as evidence of an attempt to thaw the Liberal Democrats' relationship with Ed Miliband.

In an interview for a BBC Radio 4 documentary to be broadcast on Monday night, the deputy prime minister praises Labour for embracing the possibility of sharing power while accusing David Cameron of being driven to the right by backbenchers.

Clegg said Labour had changed for the better since the last election, while the Tories had changed for the worse. "I think there's nothing like the prospect of reality in an election to get politicians to think again and the Labour party, which is a party unused to sharing power with others, is realising that it might have to," he said.

Reflecting on the role the Lib Dems would have in a possible Lib-Lab coalition, he said: "There is just no doubt in my mind that if there were a Labour-Liberal Democrat coalition, we the Liberal Democrats would absolutely insist that government would not break the bank."

Clegg's tone changed when asked about David Cameron's Tories, his partners in government. "I think the Conservative party has changed quite dramatically since we entered into coalition with them. They've become much more ideological, they've returned much more to a lot of their familiar theme tunes," he said. "I think it would be best for everybody if the Conservative party were to rediscover a talent for actually talking to mainstream voters about mainstream concerns."

Clegg's words chime with a strategy of highlighting differences with the Tories as the election approaches. A source close to Clegg conceded that the language was stronger than his previous comments on the Tories, but insisted there was no fundamental change in the party's position. "We still wish to sit between the two other parties and we are not picking one over the other," the source said.

Labour's position towards Clegg has appeared to change over recent months, from demanding Clegg's resignation as Lib Dem leader as part of any future coalition deal to paying him the occasional compliment.

In the aftermath of the 2010 election, senior Labour figures said Clegg's departure would be a precondition of any future Labour-Lib Dem coalition. Ed Miliband told the New Statesman: "Given what he [Clegg] is supporting, I think it is pretty hard to go into coalition with him."

Ed Balls similarly suggested that there was little or no prospect of Labour working with Clegg, stating as recently as September 2012: "Nick Clegg made his decisions and I think the way he's gone about his politics makes things very difficult [to form a coalition with him]."

It has been thought that just as Clegg demanded Gordon Brown's head in 2010, so Labour would demand his if it won in 2015. But in January this year, Balls surprised many by saying he would be prepared to work with Clegg in 2015, and described him as a person of "integrity". Labour's abrasive shadow chancellor even claimed that the two of them had recently had a "very friendly and warm chat".

Balls comments prompted some Labour backbenchers to claim that Clegg's dismissal should remain a condition of any future coalition, not least because he remains the least popular of all three leaders, according to recent polls.

Sources close to Miliband dismissed Clegg's latest overtures as insignificant. "Our view of the Lib Dems is clear. They have not acted as a brake on the Tories, but as a prop."

In the documentary, the former Lib Dem minister Jeremy Browne is critical of Clegg's positioning and strategy going into the next election. Browne said: "I have some unease that we are trying to pitch ourselves as a party that splits the difference between the other two … there's a sense of insipid centrism that is reassuringly unthreatening to people. That's not the same as liberalism."

Chris Huhne, the former energy secretary, criticises Cameron and George Osborne, the chancellor, for their tactics during the referendum on electoral reform, which divided most Tories and Lib Dems. Huhne said: "They went out and put out leaflets or sanctioned leaflets which were personally attacking Nick [Clegg] for going back on the tuition fees pledge, even though they had actually insisted on that as part of the coalition agreement and making the coalition work."

Shirley Williams, a former leader of the Liberal Democrats in the Lords, criticises Clegg for surrounding himself with inexperienced advisers. "I think he hasn't really sufficiently used effectively the experience and knowledge of some of the more senior members of the party … Nick surrounds himself with people his own age or considerably younger. He's got some good and some frighteningly bad as the case may be. And I think that that's unwise," she said.

But the former Lib Dem leader Lord Ashdown said: "He is, I think, the most gifted and able politician in British politics."

Clegg's words come after his party endured a dreadful week. The Lib Dems lost their deposit with less than 5% of the vote at the Wythenshawe byelection, and party donors Sudhir and Bhanu Choudhrie were arrested as part of a Serious Fraud Office corruption investigation focused on the manufacturer Rolls-Royce. The pair deny any wrongdoing.

Part two of Nick Clegg – the Liberal Who Came to Power, a two-part series on Radio 4, airs at 8pm on Monday.