Nick Clegg has accused the chancellor, George Osborne, of balancing the books “on the backs of the poor” in an attack on his coalition partner’s austerity measures.

Speaking ahead of the Liberal Democrat conference in Glasgow, the deputy prime minister said the Tories’ claim of “compassionate conservatism is dead”.

The Conservatives were widely viewed to have stolen a march on the Liberal Democrats last week by pledging to increase the personal tax allowance to £12,500.

Clegg told the Times: “You cannot say you are a compassionate Conservative if you say you will balance the books on the backs of the poor.

“To say you are not expecting the wealthy to pay a single [extra] penny towards getting rid of the deficit is extraordinary. It is an economically illiterate thing to say.”

Clegg said the Tories had “mutated almost out of recognition” since the coalition agreement was struck after the 2010 election.

“We went in with partners who told us they were green, but they are not. They told me they weren’t going to bang on about Europe, but it’s all they bang on about. They said they believed in civil liberties and they want to trash them.

“I can understand why they have done it. They are in a complete blind panic about Ukip, but I like to think we have not raced across the political spectrum like that.”

He also hit back at Theresa May, the home secretary, who used her conference speech to accuse the Lib Dem leader of putting children’s lives at risk by blocking the so-called snooper’s charter. “It’s ludicrous,” Clegg told The Times. “At the beginning of the coalition, there was a feeling that we were going to challenge the endless tub-thumping illiberalism of Labour. There was lots of ‘we will do that together’. Now, four years later, we have the home secretary telling me I am responsible for children dying.”

Despite polling that suggested as many as half of the Liberal Democrats’ 56 MPs could lose their seats in an election just seven months away, Clegg remained positive about the value of the coaltion.

“We all knew this was going to hurt politically … of course it’s damaged us but the longer-term damage to liberalism would be much more grave if we had said we didn’t want to get our hands dirty.”

David Laws, the Lib Dem education minister responsible for the party’s election manifesto, told the Financial Times that the Tories’ lurch to the right had opened up a “massive amount” of political ground for liberals ahead of the election, adding that the party had recovered from setbacks in the past.

A poll of 735 party members for the Liberal Democrat Voice website showed that 80% continued to support the coalition, but that two-thirds feared the party would have fewer than 40 MPs after the election.

Clegg has set himself on a potential collision course with party activists by backing new airport runways. The deputy prime minister said technological advances would help to negate the environmental impact of flying and pave the way for airport expansion.

The Lib Dems have previously insisted there would be no airport expansion in the south-east of England “because of local issues of air and noise pollution”, but the party’s position on Gatwick could be softening. “I do happen to think the environmental impact can … be consistent with some form of airport expansion given the rapid improvement in environmental performance of modern aircraft,” Clegg said.

The party will vote this weekend on manifesto plans to commit an extra £1bn in NHS funding over two years.

Danny Alexander, the chief secretary to the Treasury, said the money, to be delivered between 2016 and 2018, would be in addition to the existing ring-fenced commitment.

He also announced that extra investment for the health service would be the party’s priority in the coalition negotiations running up to this year’s autumn statement, scheduled for 3 December.