Tim Farron will use his first conference appearance as Liberal Democrat leader to insist that his party can present the only "credible" alternative to the Tories.
Mr Farron, who has claimed that Jeremy Corbyn's victory has led to contact from Labour figures "distressed" about the left-winger's election, will openly call on liberals in other parties to defect.
The party will gather in Bournemouth on Saturday to discuss how to move forward following the electoral mauling which saw it lose all but eight of its MPs in May, triggering Nick Clegg's resignation.
Mr Farron will attempt to rally the party's activists, insisting that the surge in membership since the election is the sign of a fightback.
He will use his appearance at the conference's opening rally to say: "Britain is teeming with liberals, some of them are not yet in the Liberal Democrats. Some of them are in other parties. But we are their home."
The party's membership has grown by 20,000 to 61,000 since the election, with a record 2,500 members expected at the conference.
Highlighting the new members, Mr Farron will say: "By their simple act of joining us they proclaim that fear and division will not win, that liberalism can and must win."
The Lib Dems believe that Mr Corbyn's victory and the Tory policy agenda since the end of the coalition means there is a space in the centre ground they can fill.
In a sign that he does not believe Labour can win the next election under Mr Corbyn, Mr Farron will say: "We once again see the prospect of a decade or more of Tory rule, and it fills us with dread.
"So we have no right to sit in a comfort zone, every family whose home is at risk because the Tories undermine the recovery that we built; every desperate refugee turned back at the port; every business facing ruin if we leave the EU, every one of them is depending on us.
"We are liberals, we correctly talk a lot about rights, but we have a duty. Our duty is to claim the mantle of the credible progressive alternative to the Tories."
Writing in the Guardian, Mr Farron set out how he believes that the election of Mr Corbyn provides an opportunity to revive his party by wooing centre-left moderates.
He said Mr Corbyn's victory "potentially changes everything" for the Lib Dems' fortunes.
"Over the past few days I have received a number of messages and calls from friends within the Labour Party distressed by the direction that their party is taking.
"To the right, I have talked to a new Conservative MP taken aback by the attitudes that they have encountered within their own party."
The result, he said, was "the opening up of a massive space in the centre ground of British politics, for sensible, moderate progressives who are opposed to what the Conservatives are doing, but cannot bring themselves to support a party of the hard left".
The Lib Dem leader, who will face a test of his authority at the conference by opposing a motion calling for the UK to give up its nuclear deterrent, risks another row with the grassroots over aviation policy after indicating he could back expansion of airports around London.
Party activists had previously ruled out airport expansion in south-east England.
But while Mr Farron maintained his opposition to an extra runway at Heathrow he said he was "open-minded" about other options for increasing capacity around the capital.
Lib Dem activists last year rejected proposals to allow the development of new runways away from Heathrow despite pleas from the party's hierarchy.
In a sign Mr Farron would be prepared to reopen the row, he told the Evening Standard: "I'm utterly opposed to Heathrow expansion, the party's utterly opposed to Heathrow expansion.
"It tends to be opposed to other expansion within London and the South-East but I think it's important that we carry on listening to the case.
"That promise over Heathrow must stay. But I'm open-minded about other expansions and other options for London and the South-East."
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