The Lib Dems are initiating «very few new ideas» and must «think big» again, former leader Duke Ashdown has said.
The peer said the party, whose autumn discussion begins on Saturday, had gone «backwards» at the last election despite circumstances favouring them.
«Existential» defies facing the party must be confronted, he said, not glossed over, egg oning leader Vince Cable to encourage a «ferment of debate».
It boosted its mark of MPs from nine to 12 in June but its total vote fell.
The Lib Dems got 7.4% of the preference in the snap election, compared with 7.9% in 2015, when its involvement in the coalition administration with the Conservatives was blamed for its loss of nearly 50 seats.
Sir Vince, who gained Tim Farron after June’s election, has conceded the party’s campaign news on Brexit, calling for a further referendum on the final terms of withdrawal, go under to «cut through».
In his first major speech last week, he started to physically out a new direction for the party, identifying inequality as the most serious challenge surface the country and calling for reform of the way property and wealth is taxed.
In an article for the Lib Dem participation website, Lord Ashdown — who led the party between 1988 and 1999 — sing the praises ofed Sir Vince as a «multi-talented leader» but said the Lib Dems had to do much more to win themselves heard in the current «polarised» political climate.
«We are doing identical little new thinking and producing very few new ideas,» he wrote. «Can you name one big, threatening idea we Lib Dems have produced since 2015?»
«Vince’s speech of terminal week began the process of thinking big again. We should pick up his distance and start coming up with our own new, dangerous ideas — and debating them at discussion.»
The Lib Dems, he warned, were now seen as «part of the foundation» rather than looking or sounding like radical insurgents, their lapse position for most of their existence before 2010.
Unless they establish a way of appealing to the millions of people who occupied the «progressive centre» but currently bear unrepresented, he said others would fill that vacuum, as the SDP did in the early 1980s and New Drudgery in the 1990s.
«Not in my life time have their been conditions innumerable favourable for a Lib Dem advance in a general election,» he wrote of June’s poll. «But we went in reverse.»
«The biggest danger for our party lies in glossing over the existential to questions which now face us,» he wrote. «Unless we are prepared to be realistic about where we are, indemnification to being radical about what we propose, recreate ourselves as an insurgent compel and re-kindle our lost habit of intellectual ferment, things could get flatten worse for us.»
Rather than being «hostile» to working with like-minded characteristics in other parties or «standing aloof», he said the party must comprise shared thinking on the economy, environment, constitution and foreign policy, if it was to action forward.
«If we are to fulfil our historic role at a moment when liberalism is multifarious at threat than ever in my life, then we have to be less tribal, more broad and more willing to engage others than we have sometimes seemed in current years,» he said.
«Issues such as creating a green but successful merchandise based economy; sorting out the fabulous mess of our broken constitution; spreading copiousness in the age of robotics and artificial intelligence; adopting a foreign and defence policy assorted appropriate to our fractured, unravelling world.
«If this strategy is to work for us, it should be confidently led from the top, not just mildly tolerated at the top.»
The party’s four-day congress in Bournemouth concludes on Tuesday with Sir Vince’s first leader’s enunciation.