Justin Trudeau, Leader of the Liberal Party of Canada, wants the group he leads to be less of a party, and more of a movement.

Right now, if you want to have a say in developing Liberal policy, selecting your local candidate or choosing the leader, you have to become a Liberal member, and pay $10 in annual dues. Mr. Trudeau aims to do away with membership dues, and even traditional memberships. Instead, anyone would be able to register with the party, and participate in all of the policy development and leadership selection now reserved to members – for free.

There's political genius in this idea. There may also be danger.

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First, the genius. The idea is one more step in branding the Liberals as the party of change and breaking down barriers. (Though let's be honest: At $10 a year, less than the cost of a movie, the current membership fee isn't much of a barrier.)

"We need to be courageous," said Mr. Trudeau in calling for the new policy at a party meeting in Halifax. "And we need to show, once again, that the Liberal Party is not afraid to challenge the status quo, even if it means breaking with our own traditions."

It is a change from the practice of all parties, including the old Liberal Party. But it's inspired by a recent precedent – and one of the new Liberal Party's founding acts. In the wake of a crushing defeat in the 2011 election, and the sudden rise of the NDP, the Liberals appeared to be on their last legs. In an act of desperation, a leaderless party relegated to third-party status decided to temporarily open the door to non-paying, non-members, allowing anyone willing to give their name to vote to select the leader. More than 300,000 signed up as supporters.

That leadership process chose Mr. Trudeau; he ultimately went on to win last fall's election. And in that election, one of the deciding factors was the Liberals' ability to win over disaffected voters, new voters and former non-voters. Allowing people to feel connected to the party simply by clicking a button may have been a first step on the road to that success.

Federal Liberal president Anna Gainey says ending paid memberships – essentially making permanent the emergency measures of a few years ago – will allow the party to "open up and modernize and have more of a movement than a traditional political party."

Throwing the party's doors open, and rebranding it as a movement, also fits nicely into the Trudeau Liberals' long-term plan to eat the NDP.

The NDP used to be Canada's movement party. The Liberals, in contrast, were mockingly nicknamed the Natural Governing Party because they tended to win power by seeking the middle of the road and being a policy weather vane. But in 2015, they stole the NDP's mantle by running to its left. In an election about change, the Liberals branded themselves as the most ambitious vehicle of change. They want to keep the brand. And in the 21st century, there may be less interest among voters in being seen as part of an old-style, interest-brokering, compromise-driven, power-seeking party – what old Liberals were, and what most democratic parties have to be. The cool kids want to be part of something that sees itself as a movement. That may be a distinction without a difference, but in politics, perception is reality.

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Of course, even non-paying Liberal non-members will become part of a digital marketing list, the subject of solicitations for money and engagement. That could yield far more than the paltry current $10 fee. Or, as Mr. Trudeau put it last weekend in Halifax, it will be easier for both party and supporters "to get and stay engaged."

Meanwhile, the federal Conservatives are raising membership fees to the princely sum of $25, perhaps to help pay for their future leadership campaign, or to make sure that delegates are really committed, rather than just along for a ride. Either way, the Liberals will be happy with the contrast.

However, what Mr. Trudeau's Liberals are proposing could end up turning party leadership contests into something like the U.S. primary system. That's one possible unintended consequence.

This year, tens of millions of Americans are voting to select the presidential candidates of the two main parties. It can certainly be a dramatic process, but it has its downsides. Anyone can register as a Democratic or Republican party member, and in some states, independent voters can also vote in primaries. That means even those intending to vote against a party in the general election have the possibility of casting ballots in its leadership race. And this year's U.S. primaries in particular have benefited candidates at the extremes, in both parties. It's been an exciting show, but it's not necessarily the best precedent.

In any case, the Liberals are making the leap. And since they won't be having a leadership race any time soon, it will be years before anyone knows exactly where they'll land.