Boris Johnson has reshaped the Conservative party to his own “ugly” values, Ed Davey has argued, using his speech to the Liberal Democrat conference to make an open appeal to traditional Tory voters who dislike the prime minister and his populist ways.

Addressing a crowd of about 200 activists for the sole in-person event of an otherwise virtual annual gathering of the party, the Lib Dem leader repeatedly emphasised what he said was the vulnerability of Conservative seats in the so-called blue wall.

The idea that the Lib Dems can challenge the Tories in dozens of prosperous commuter belt seats, mainly in the south of England, gained prominence in June when they won the Chesham and Amersham byelection, overturning a Conservative majority of more than 16,000.

In a speech featuring few new policy suggestions beyond the idea of diverting some of the post-Covid educational catch-up budget directly to parents for them to decide how it is used, Davey targeted Johnson and his ministers, citing what he called the prime minister’s “total lack of shame or decency”.

“The truth is that over the last few years Boris Johnson has remade the Conservative party in his own image, and it is an ugly, ugly sight: his casual disregard for facts or truth, his trail of broken promises, from no border in the Irish Sea to no tax rises,” Davey said.

“Boris Johnson is not a prime minister worthy of our great United Kingdom. His Conservatives are not a government worthy of the British people.”

Davey said the Chesham and Amersham result highlighted a “groundswell of frustration and discontent from people who feel ignored and taken for granted” by Johnson’s government, saying large numbers of lifetime Tory voters felt “betrayed”.

He said: “They just don’t feel that Boris Johnson represents them or shares their values. They’re not convinced the prime minister is competent – or, worse still, decent.”

Davey’s counter-offer to such voters – so far more of a broad, values-based concept than one containing many new policies – is something he called “the British fair deal”, the idea that people who work hard and pay taxes will, in return, receive decent public services.

The main new idea centred on efforts to help pupils catch up after the disruption from the Covid pandemic, with Davey saying the Lib Dems would push for the full £15bn package called for by Sir Kevan Collins, the government’s adviser on the issue who resigned when his package was rejected in favour of one about a tenth of the size.

“First, schools should be free to spend their Covid cash as they see best,” Davey said. “And the second reform: a third of the education catch-up fund should go to parents directly – in the most radical empowering of parents ever.”

This three-year plan would allow parents to use their vouchers at their school, but also spend them on private tuition or on outside classes in subjects such as music or sport.

Tory voters ‘are not convinced Johnson is competent –or, worse still, decent’, according to Davey.

The value of these vouchers would be doubled or tripled for children in care, those with special educational needs, or those from disadvantaged backgrounds.

Davey, who has sought to rebrand the Lib Dems as the “party of carers”, also lambasted Johnson’s new increase in national insurance contributions to pay for the NHS and care, saying the lack of a coherent plan for care was “just another of his broken promises”.

He repeatedly raised decisions by Johnson that Lib Dem strategists believe play badly among more traditional Tory voters in blue wall seats, such as cutting the foreign aid budget, something Davey called “indecent, immoral and wrong”.

To emphasise the point even more, in a warm-up just before Davey’s speech, Layla Moran, the party’s foreign affairs spokesperson, and the party treasurer, Tilly McAuliffe, knocked down a wall of blue cardboard boxes, a prop first used in victory celebrations at Chesham and Amersham.

“Let us bring the blue wall down,” Davey said, adding that such victories could take place in “Cambridgeshire and the Cotswolds, Stockport and Surrey, Hampshire and Hertfordshire”.

There is debate among psephologists about how many blue wall seats the Lib Dems could potentially take. However, one thing is clear: the win in Chesham and Amersham provided a huge boost to Davey, who took over the party in the wake of its disastrous 2019 election campaign, one that was preceded by much optimism and bullish predictions about seat gains.

Sarah Green, the victorious byelection candidate, was among Lib Dems MPs in the front row of the speech venue, a conference room in Canary Wharf, east London, and was highlighted by Davey for applause.

Davey stressed the Lib Dems’ key role in being able to remove Johnson. “Make no mistake: the electoral arithmetic is clear,” he said. “These Conservatives can’t be defeated next time unless we Liberal Democrats win Tory seats. Boris Johnson will stay in Downing Street unless we throw him out.”