After his party's weekly caucus meeting, Mulcair said the real threat of terrorism requires responsible measures, not the "dangerous, vague, ineffective" Bill C-51.
"We in the NDP are going to fight it," Mulcair told reporters. "The truth is we cannot protect our freedoms by combating them."
Bill C-51, which would give more powers to law enforcement and intelligence agencies, is up for second reading and debate today.
The bill, introduced by Prime Minister Stephen Harper in January, would allow the Canadian Security Intelligence Service (CSIS) to "disrupt" or "counter-message" terror activity inside and outside of Canada and make "promotion" or "intentional advocacy" of a specific terrorism offence illegal instead of the current laws against counselling or actively encouraging one.
It would also lower the threshold for arrest to when law enforcement agencies think a terrorist act "will be carried out" to when they believe one "may be carried out."
The proposal has been criticized for its potential effects on free speech, privacy and the ability to protest, but former CSIS assistant director of intelligence Ray Boisvert told CBC News the day the bill was announced that it's "very, very necessary" to give more power to disrupt when there is an immediate threat to life.
Mulcair said the fact the bill was presented at a campaign-style event in Richmond Hill instead of in Parliament shows the bill is politically motivated.
"What they're trying to do is posture and position," he said.
Liberal Leader Justin Trudeau has already said his party will support the bill, although he says he will push for greater oversight of CSIS. But Trudeau says the bill's failings aren't enough for the party to vote against C-51.
Green Party Leader Elizabeth May is vehemently opposed to the bill and says she'll present amendments, but that the best way to deal with it would be to scrap it.