Sir Vince Cable is planning an overhaul of the Liberal Democrats in an attempt to turn the party into a “Movement for Moderates”.
The veteran politician will outline proposals for major reform of his party to allow new supporters to join for free and have a say in future leadership contests.
The plans will be seen as an attempt by the Lib Dems to mirror the success of left-wing pressure group Momentum, which has attracted thousands of new supporters to Labour under Jeremy Corbyn’s leadership.
Labour’s membership has soared in recent years since party reforms allowed anyone to sign up to vote in a leadership contest by becoming a registered supporter for a small fee.
As of April this year, Labour had 540,000 members compared to 125,500 SNP members, 124,000 Conservative members and 99,200 Lib Dem members.
In a speech on Friday, Sir Vince will describe how he wants to create a formal link with online activists who “loosely identify” with the Lib Dems.
The 75-year-old, who has led his party since July last year, also wants to connect with “the vast swathe of voters in the centre ground whom we are yet to persuade”.
He is expected to say: “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.
“Whatever rights our new supporters gain, we as a party aim to be in constant conversation with them, engaging them in campaigns and urging them to begin campaigns of their own.
“I want these not to be just about stopping things but about growing support for the things that matter to Liberal Democrat voters, and to the vast swathe of voters in the centre ground whom we are yet to persuade.”
Despite taking an anti-Brexit position, the Lib Dems have failed to capitalise on pro-EU sentiment among large numbers of voters.
The party is yet to recapture the same popularity it enjoyed prior to entering the coalition government in 2010.
But, Sir Vince will point to how activists joining together through groups such as More United, 38 Degrees and Change.org have shown the Lib Dems how they “can engage hundreds of thousands of people online”.
He is expected to add: “We are a centre-ground, pro-European, liberal and social democratic party, welcoming like-minded supporters.
“This will be a Movement for Moderates.”
A Lib Dem source recently confirmed Sir Vince’s plans for a radical shake-up of party rules could allow a non-MP to succeed him as leader.
But they dismissed reports Sir Vince will use Friday’s speech to set out a timetable for him stepping down as leader.
Businesswoman and prominent anti-Brexit campaigner Gina Miller ruled out a bid for the Lib Dem leadership following the emergence of Sir Vince’s proposals for future leadership contests.