Prime Minister Justin Trudeau wants to transform the Liberal Party from a members-only club into a more inclusive – and free – political movement.

A proposal, adopted at a meeting of the party's national board over the weekend, would do away with the long-standing policy that only dues-paying, card-carrying Liberals can get involved in party activities. Delegates to the party's national convention in Winnipeg in May are slated to vote on a new party constitution, including moving to free registration.

"Our existing constitution makes it tough for Liberals to get and stay engaged," Mr. Trudeau said, speaking to supporters in Halifax on the weekend.

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The proposal emerged from a recent survey of 2,100 Liberals in which 96 per cent endorsed "opening up party membership more broadly," said Braeden Caley, a party spokesman.

The Liberals currently charge $10 a year for memberships. But during the 2013 race, when the party chose Mr. Trudeau as leader, non-members were invited to vote without first buying a membership. The result was that roughly 300,000 people registered to vote, many from outside the party.

"We've tried the supporter system and it was a huge success," Liberal Party president Anna Gainey said. "I believe that as we continue to open up and modernize and have more of a movement than a traditional political party, that is a natural progression of that."

The move comes as the NDP, which is holding its convention this week in Edmonton, is also exploring new ways to broaden its base of support. The Conservative Party of Canada recently raised its membership dues to $25 and will limit voting in its coming leadership race to those who have been party members a minimum of six months.

NDP national director Karl Bélanger said the party is looking to grassroots members, not the party brass, for ideas. "It's paradoxical that this [Liberal] proposal is coming from the top, not the membership," Mr. Bélanger said. "That's not the way we operate in the NDP. Grassroots mean you let the grassroots decide and come up with proposals."

The cost of NDP memberships currently runs from nothing to $25 a year, depending on where members live (they're free in Newfoundland and $1 in Alberta), their age and whether they're employed.

Conservative Party spokesman Jake Enwright declined to comment on Sunday.

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Mr. Trudeau met with the party national board for nearly three hours over the weekend to talk about creating a single party constitution, eliminating the 18 it has now, and to look at ways of modernizing the party structure – using more technology and developing a more open process for developing policy.

The proposed new constitution would be shorter than the current 81-page document and give more flexibility to the national board – which includes elected riding presidents from across the country – to adapt and modernize party procedures in a timely way. That includes its cumbersome policy development process. The constitution currently stipulates that policy resolutions are to be put forward by riding associations for consideration at annual general meetings of the various provincial and territorial wings of the party. Priority resolutions chosen at those meetings are then debated and voted on at the national party's biennial conventions.

The process "does not respond to the pace of life in the digital age," Ms. Gainey said.

When Liberals agreed four years ago to let supporters vote in leadership contests, they stopped short of similarly opening up the process for nominating candidates to run for the party in elections. Many worried that a wide-open nomination process at the riding level would be too easy to manipulate by political opponents.

But Ms. Gainey said rank-and-file Liberals, having seen the benefits of opening up the leadership process, are now behind the idea of doing away with exclusive privileges for paid members altogether. The party has wasted a lot of time and energy renewing memberships every year and going through the cumbersome riding-by-riding process of electing delegates to attend party conventions, which are never filled to capacity, she said.

With a report from The Canadian Press