On Tuesday, Garneau supported Trudeau's plea to the Liberal Party to extend the registration deadline for voters in the leadership race, and sources have told CBC News that the deadline will be moved to give more time for supporters who are registering by mail, rather than by email.
The former astronaut's support for the registration date change was a sign perhaps that he intended to get behind Trudeau.
Wednesday at a news conference in Ottawa, Garneau said, "It's a fait accompli," about the likelihood of Trudeau winning the race.
"I cannot see the numbers changing because he has an overwhelming lead," the MP for the Montreal riding of Westmount-Ville-Marie said.
The odds were long but not impossible when he entered the race, Garneau said. He pointed to internal polling showing he is running second, although others have said he is in third or fourth place.
Garneau said his poll, or survey, conducted last week, contacted 40,000 supporters or party members who intend to vote in the race. Six thousand replied and of those, he said, Trudeau had 72 per cent support, he had 15 per cent, Joyce Murray had 7.4 percent and Martha Hall Findlay had 5.2 percent.
Garneau gave no indication of a margin of error for the survey, or how it was weighted geographically. Each riding is worth 100 points, meaning 50 points in a remote rural riding are worth as much to a candidate as thousands in a large urban riding.
"I believe in numbers, OK," said Garneau, who has an advanced engineering degree. "The numbers don't lie."
Garneau challenged Trudeau
Garneau's support of Trudeau is somewhat of a surprise, given that Garneau suggested strongly the Liberal MP from Papineau lacked substance and didn't have the resume to be a leader and compete against Stephen Harper.
"The leadership of the Liberal party is too important a position to be handed to an untested candidate who is hiding behind a carefully crafted public relations campaign," Garneau said at a news conference Feb. 25, referring to Trudeau.
Garneau had also challenged Trudeau to a one-on-one debate, which Trudeau declined. On Wednesday Garneau said, "It would have been nice for me, because I said so at the time, but he chose not to do it, and that's the way it is."
Asked by reporters about how he could now support Trudeau after he had criticized his vagueness about policy, Garneau replied, " I've been very constructive in this leadership race, and I've always believed in a rigourous race and I think that's a perfectly good thing. And Justin has risen to the occasion."
All along, Garneau has been wary about revealing how many supporters he had personally signed up, and how much money he has been fundraising, other than to say it would be revealed "in due course."
However the claim from the Trudeau camp that its campaign has garnered more than 160,000 supporters was an indicator of how far Trudeau has pulled ahead of other candidates, as well as the amount of money he has raised.
Elections Canada reported on its website last month that Trudeau had fundraised in excess of $600,000. Since then, estimates have ranged from $1 million to $2 million, far more than he needs to run his campaign. The excess money will be funnelled back to the Liberal Party.
On Wednesday, Garneau, a former Navy captain, described himself as a "loyal soldier" who would be happy to support Justin Trudeau as Liberal leader. "You're going to see my face around for a long time," Garneau said.
Shortly after the news conference ended, Justin Trudeau tweeted, "Thank you @MarcGarneau for your support and for a lifetime of service to Canadians. Lots of work still to do, together!"
Interim Liberal Leader Bob Rae said he wouldn't comment on who's running first or last, but said of Garneau, "He told me that he just made a very practical calculation and said, 'you know, I don't think I'm going to get over the finish line.' and he's not somebody who's interested in making a symbolic gesture, so he decided to pull back."
Other candidates praise Garneau, question his math
Vancouver MP Joyce Murray said, in an interview with CBC Radio, "It sounds as if he [Garneau] has drawn a conclusion that there is only one possible winner...I totally disagree with his conclusion." Murray also questioned the validity of Garneau's poll, saying it was a robocall that told recipients to press 1 for Justin Trudeau. "We don't give any credit to those numbers," she said.
Lately, there have been indications that Garneau, had he stayed in the race, might place third after Murray. Murray is said to be attracting large numbers of supporters due to her pitch for electoral co-operation with the NDP and the Green Party
Martha Hall Findlay, a former MP now running for leader, said it was important to have had Garneau in the race, and that he "represented a serious, substantive, experienced choice for the Party, as do I."
As for Garneau's poll, which has her running fourth, she disputed it: "It's not just the numbers, it's the math. There are some ridings with thousands of registered voters — but each one of those ridings is worth 100 points." She said that her campaign has been operating "strategically", trying to win supporters in every riding.
Leadership candidate Martin Cauchon, a former Chretien cabinet minister, said Wednesday that the race is just beginning, even though it has been going on since November 14, 2012. "When you look at my campaign, you look at me for example, I have the experience, the vision and the determination, of course," he said.
Cauchon also said he was surprised Garneau was backing Trudeau, considering he was "on the attack vis-a-vis Justin over the past few, past few weeks."
Candidate Karen McCrimmon said, in a statement, that she regretted Garneau's decision to withdraw from the race, but she too questioned Garneau's assertion that Trudeau will win. "A preferential ballot equally weighted across 308 ridings could provide surprising results," she said.
At his final news conference as a leadership candidate, Garneau seemed relaxed and happy, and welcomed a question about astronaut Chris Hadfield who is about to take command of the International Space Station. "I wish Twitter had existed during my time in space. This might have had a different outcome," a beaming Garneau said.
There are still seven candidates left in the leadership race. The results will be announced in Ottawa on April 14. Voting will take place April 7-14.
Garneau was first elected in 2008.