Trudeau told supporters at Emmanuel Dubourg's campaign headquarters that the NDP of Tom Mulcair is much different from the one that stunned pundits during the 2011 election.
"Make no mistake, the NDP is no longer the hopeful, optimistic party of Jack Layton," Trudeau said. "It is the negative, divisive party of Thomas Mulcair. Because it is the Liberal party tonight that proved hope is stronger than fear, that positive politics can and should win out over negative."
Trudeau's reference to hope being "stronger than fear" naturally reminded many of words found in Layton's death-bed letter to Canadians.
"My friends, love is better than anger. Hope is better than fear. Optimism is better than despair," Layton wrote. "So let us be loving, hopeful and optimistic. And we’ll change the world."
Many New Democrats, including Mulcair, were outraged that Trudeau would attempt to use their former leader's words against them.
"That Justin Trudeau would use Jack Layton's dying words as a political tool says everything that needs to be said about Justin Trudeau's judgment and character," Mulcair said Tuesday.
But the Liberal leader did not back down, telling reporters that it's the NDP that may need to ask some questions about how it campaigns.
"I am, as many people are, inspired by Jack Layton's legacy and the way he approached politics," Trudeau said. "And I'm very, very proud that across the country, in all four byelections, the Liberal party ran a positive campaign that was focused on bringing people together and not on attacking or smearing our opponents."
Ian Gillespie, a special advisor to Mulcair, took to Twitter after Trudeau's speech Monday night to say it was disgraceful to quote Layton.
But some Liberals suggested online that it was Layton who paraphrased former Liberal prime minister Wilfrid Laurier, who once said: "Let me tell you that for the solution of these problems you have a safe guide, an unfailing light if you remember that faith is better than doubt and love is better than hate."
The Bourassa byelection, much like the race in Toronto Centre, was largely a battle between Liberals and New Democrats.
The NDP created a "Club Privilege" website to mock Dubourg, a former Quebec MNA, for taking a $100,000 allowance after he quit his seat this year to run federally.
Trudeau pointed out that Mulcair received a similar payment when he left Quebec politics in 2007.
The campaign got somewhat nasty at points. Liberal campaign staffers were caught on tape removing "Club Privilege" signs posted directly in front of Dubourg's office.
But as you can see in the gallery below, some NDP supporters felt Trudeau's reference to Layton's legacy went too far.
UP NEXT: Trudeau Glam Shots
UP NEXT: Memorable Trudeau Photos
Come At Me, Bro
Justin Trudeau trains at Pan Am Boxing Club in Winnipeg on Friday Feb. 1, 2013.
Justin Trudeau & co. making faces.
Justin Trudeau splits his pants while pushing the "scrum machine" in support of Prostate Cancer Canada in Toronto Thursday, July 21, 2011.
Justin Trudeau gets his geek on at Montreal Comiccon in September 2012.
So Long 'Stache
Justin Trudeau has his moustache shaved off to raise money for the Judy LaMarsh Fund, that supports female candidates, at the Liberal Party convention in Ottawa on Saturday, January 14, 2012.
Coming For MacKay
Minister of National Defence Peter MacKay (left) is chased by Liberal MP Justin Trudeau in a motorized wheelchair during a wheelchair race relay on Parliament Hill in Ottawa on Wednesday, May 12, 2010. Twenty-five MPs and senators used a wheelchair for the day in support of the Canadian Paraplegic Association's Spinal Cord Injury and CPA awareness month.
All For One, One For All
Justin Trudeau all dressed up for the Montreal Movember Gala in 2010.
Pierre Trudeau's sons, Sacha, left, and Justin, tackle their mother's paperboy in Ottawa in this undated photo.
'Family... And A Cow.'
He Can Certainly Take A Punch
Alexandre (Sacha) Trudeau delivers a right hook to his older brother Justin during a play fight in 1980 at Ottawa airport as the boys await a flight with the return of their father, then-prime minister, Pierre Trudeau.
Be Honest With Me, Who's Cuter?
Justin Trudeau strikes a pose with an adorable baby.
A Very Furry Christmas
Justin Trudeau poses with his family on his 2010 Christmas card.
Former Liberal MP Ken Dryden, left, and Justin Trudeau play table hockey as they visit Sun Youth, a community organization, Monday, Jan. 14, 2008 in Montreal.
Do you feel the Liberal leader's words were in poor taste? Tell us in the comments.
With previous files