Justin Trudeau has dissolved a vital bond between senators and the Liberal Party that appointed them, expelling dozens of members of his caucus in a dramatic attempt to reshape the Senate without a constitutional debate.

The move, which leaves 32 members of the Senate sitting as independents, was accompanied by a proposal for a non-partisan appointment process that would take sole decision rights away from the Prime Minister – a powerful instrument that heads of government have held since Confederation.

"The Senate must be non-partisan, composed merely of thoughtful individuals … independent from any particular political brand," Mr. Trudeau said on Wednesday morning, after expelling the senators from caucus.

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For months, a widening criminal expense probe has mired the Senate in scandal and turned the unelected body into a liability for both Stephen Harper's government and the third-party Liberals, whose legacy of power remains evident in the 105-seat Senate despite diminished ranks in the House of Commons.

Mr. Trudeau's surprising decision has put renewed focus on a scandal that has claimed the Prime Minister's chief of staff and a trio of senators he appointed. The Conservatives – who have long campaigned on Senate reform but are accused of doing little to change it – have recently turned to the Supreme Court for guidance on the possibility of elections. The New Democrats, for their part, have called for outright abolition.

Mr. Trudeau, according to a party source with knowledge of the Liberal discussions, asked his advisers in the fall to present him with a third way – an option that would target patronage and partisanship, without opening the Constitution.

Conservatives dismissed the proposal as a stunt that would do nothing to make the Senate more accountable to voters. New Democrats accused Mr. Trudeau of trying to distance himself from the fallout of a looming audit into senators' expenses. And senators, who began the day as Grits and ended it as outcasts, responded with a roller-coaster mix of confusion, disappointment and unflinching support.

Many senators who were appointed as Liberals have been active in the party for years, playing critical roles in fundraising and election campaigns. So it was with some degree of alarm that they learned they were no longer welcome in caucus.

After Mr. Trudeau delivered the news, the senators met behind closed doors and emerged to say they would stick together as a Senate Liberal Caucus.

"It was a surprise announcement. We were not consulted," Senate Liberal leader James Cowan said. He added that Mr. Trudeau's ruling "set us free."

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Senator Jim Munson said the shock would wear off. "I think you have to be practical and I think you have to be accepting."

Mr. Trudeau would prefer that those senators appointed as Liberals cease to use the brand and said that, while the senators could hold party memberships, they could not be "Liberal organizers, fundraisers, activists in any form."

But some senators saw things differently.

"I'm still a Liberal senator, not independent," Senator Mobina Jaffer said. "I'm still a Liberal. I've always been a Liberal. I've been a Liberal for 38 years.... I can absolutely campaign. It doesn't stop me doing anything any other Liberal would do."

Later in the day, in the Senate chamber, things got slightly more confusing. In response to a point of order, Senate Speaker Noel Kinsella – a Conservative – ruled that the outcast senators remain both Liberals and the opposition in the Red Chamber, saying they met the threshold set out in Senate rules.

In the House of Commons, Mr. Harper mocked the turmoil: "I gather the change announced by the (Liberal) Leader today is that unelected Liberal senators will become unelected senators who happen to be Liberal."

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NDP Leader Thomas Mulcair pointed out that his party introduced a motion in October to have all senators sit as independents – a motion the Liberals joined the Conservatives in defeating.

The Liberals say they have been working on their Senate strategy for many months.

The Liberals say they consulted widely with constitutional scholars over the course of several months. The important issue, the party source said, was ensuring that they did not interfere with the work of the Supreme Court which has been asked by the Conservative government to determine whether it can unilaterally create elections for senators.

Canadians "don't want a long, rancorous, and likely pointless debate with the provinces that would distract us from focusing on more important problems," Mr. Trudeau said.