Calling for the Lib Dems to open itself up to non-members in a similar way as Labour in the leadership elections won by Jeremy Corbyn, Mr Cable will say the party’s future lies as a “political arm within Parliament” of a wider centre-ground movement.
Mr Cable has tied the Lib Dems’ fortunes to the campaign for a referendum on the terms of the UK’s Brexit deal in a bid to attract supporters who want to reverse Brexit, without a significant impact.
The Lib Dems’ poll rating sits at between 6 and 10 per cent, despite being the only major party to explicitly back the People’s Vote campaign.
Unveiling his plans in a speech at the National Liberal Club in London, Mr Cable will call for an overhaul of party rules to try and capture “the vast swathe of voters in the centre ground whom we are yet to persuade”.
“Groups like More United, 38 Degrees, Avaaz and Change.org have shown us how these regular conversations can happen, how we can engage hundreds of thousands of people online,” he is expected to say.
“I want our party to do that and to offer our movement a political arm within Parliament. So it is not just a protest group banging at the door, but a movement with a voice on the inside – our parliamentary party.”
Mr Cable will add: “We should widen membership with a new class of ‘supporters’ who pay nothing to sign up to the party’s values.
“They should enjoy a range of entitlements, including the right to vote for the leadership and to shape the party’s campaigning online.
“The Liberal Democrats already have an army of voluntary helpers and deliverers, as well as 200,000 online supporters, who loosely identify with us and campaign with us, but currently have no say in the direction of the party.”
Mr Cable could also propose that leadership candidates be permitted to stand from outside the Commons and the party altogether, according to reports.
It has also been suggested that the Lib Dem veteran will stand down in a year to 18 months in favour of a younger leader, unless there is a snap election.
The sweeping changes must be approved by the party’s existing membership at their annual conference, which is being held in Brighton next weekend.
Mr Cable’s proposals have yet to meet significant public opposition from within the party, but senior figures have expressed unease over the radical plans.
One Lib Dem MP told the Scotsman: “If you’re going to launch a 21st century movement, of course you do it in a 19th century private members’ club.”
The MP added that Jo Swinson, the East Dunbartonshire MP who is expected to take a break from maternity leave to attend today’s speech, remained the favourite to succeed Mr Cable as leader. Senior Lib Dems are agreed that the party must choose its first female leader, with Oxford West MP Layla Moran also being talked about as a possible candidate.