Trudeau got another boost later in the day when the party agreed to his request to extend the deadline for voter registration by one week, a move likely to most benefit the front-runner.
Garneau's decision followed an internal poll conducted by his campaign last week, which he said showed Trudeau has the backing of 72 per cent of Liberal members and supporters.
Garneau maintained he was a "solid second" with 15 per cent, followed by Vancouver MP Joyce Murray with just over 7 per cent and former Toronto MP Martha Hall Findlay with just over 5 per cent.
"I have done my numbers. I cannot mathematically — and I'm a person who believes in math — I cannot mathematically win," the Montreal MP told a news conference.
"I'm not into denial. The numbers indicate very clearly that Justin is the overwhelming favourite."
However, Murray said she puts no stock in Garneau's survey, the methodology of which her camp finds highly suspect. She said it won't influence her decision to continue her campaign, which has enjoyed a late burst of momentum.
"I'm not going to give it any credit," Murray said in an interview.
"I happen to completely disagree with his assessment that there's only one possible outcome to this race ... It's not a done deal."
Garneau's automated phone poll asked 6,000 Liberal supporters and members to indicate whether they favoured Garneau, Trudeau, Hall Findlay or Murray, in that order and without mentioning the other four contenders.
It was a not a survey of registered voters, the Murray camp noted, even though the outcome will depend on which campaign is best able to get supporters to register and then actually cast ballots during the week of April 6.
Murray's camp is hopeful her supporters — urged on by a host of grassroots and online advocacy groups who back her environmental credentials and her plan for electoral co-operation among progressive parties — are more committed than average about registering and voting.
Nor did Garneau's survey take into account the fact that the results will be weighted to give each riding equal clout, whether they have 1,000 registered voters or 100. Without knowing the distribution of each candidate's support, Hall Findlay pointed out it's impossible to accurately predict the outcome — although no one disputes Trudeau is in the lead.
"For those talking about polls and numbers: we're not naive, and we're fully aware of the challenge," Hall Findlay said in a statement.
However, she added: "There are some ridings with thousands of registered voters — but each one of those ridings is worth 100 points. There are also a large number of ridings with very few, many with fewer than 50. Each one of those ridings is also worth 100 points. We have run a truly national campaign, and have approached it very strategically.
Only those who register will be allowed to cast ballots during the week of April 6; so far only a third of the party's 294,000 members and supporters have done so.
The deadline for registration was supposed to be Thursday but the party has acceded to Trudeau's request and extended it to March 21.
The Trudeau team had argued that the party needs more time to send registration packages to some 100,000 people who didn't provide email addresses — the bulk of whom are Trudeau supporters — and to resolve other technical glitches that threatened to disenfranchise many others.
National membership secretary Matt Certosimo agreed, deciding that extending the deadline was consistent with the party's decision to create a new supporter class, aimed at engaging more Canadians in the selection of the leader.
"If a longer registration period facilitates a further expansion of the number of people able to participate, without putting at risk the security of the vote, then the deadline should be as late as it can possibly be," he said in his ruling on the matter.
Extending registration was opposed strenuously by Murray's camp and to a lesser degree by Hall Findlay and Martin Cauchon.
In a statement, Murray accepted the party's decision but reiterated her concern that the party appears to be changing the rules to accommodate the front-runner.
"We all agreed to a set of parameters and rules that would govern the process and our campaigns," she said.
"Changing them at this late stage has inherent risks particularly when only one of the campaigns asserts that changes need to be made."
In the interview, Murray said she was surprised by Garneau's decision to endorse Trudeau, given that he has repeatedly said a coronation is not in the party's best interests and has accused the front-runner of lacking policy depth, experience and leadership credentials.
Garneau insisted Wednesday that his criticism of Trudeau was "constructive," that the front-runner "has risen to the occasion" and proven an unmatched ability to rally people to the party.
"One must render unto Caesar that which is due to Caesar," he said, adding that he intends to remain a "loyal soldier" under the new leader.
Garneau's departure leaves seven candidates in the running: Trudeau, Murray, Hall Findlay, Cauchon, Deborah Coyne, David Bertschi and Karen McCrimmon.
Garneau, an MP since 2008, kicked off his campaign last November, stressing the economy as his key issue.
Voting for the Liberal leadership will be conducted online and by phone early next month, with the winner to be announced on April 14.
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