The NDP leader's announcement underscored the degree to which the controversial bill has become a political football in an election year.
Prime Minister Stephen Harper has played on popular support for cracking down on extremists following the murder of two Canadian soldiers last fall, portraying opponents of the bill as soft on terrorism.
Hoping to inoculate himself from that charge, Justin Trudeau has said Liberal MPs will support the bill even if they fail to win amendments to ensure parliamentary oversight and review of the new powers proposed for security services.
A Liberal government would fix those flaws should it win the election scheduled for October, Trudeau says.
And now NDP strategists are hoping Mulcair's unequivocal opposition will help lure back progressive voters who've drifted away from the NDP since the last election and towards the resurgent Liberals. Wooing back those "red-orange" switch voters is the top priority for the NDP over the next eight months.
Mulcair labelled Trudeau's stance on the bill "pathetic" and urged the Liberals to reconsider.
"This bill merits real debate," Mulcair said.
"Mr. Harper and the Conservatives have intimidated the Liberals into supporting this deeply flawed legislation. We in the NDP are going to fight it."
He likened his party's stand to former leader Tommy Douglas' courageous opposition to the War Measures Act, invoked by Trudeau's father Pierre, the Liberal prime minister during the October 1970 FLQ crisis.
Trudeau countered by chiding Mulcair for making "personal attacks" on an issue that should be debated "in a respectful and non-partisan fashion."
But he then took his own shot at the NDP.
"The fact is the NDP has not once in its history supported strengthening anti-terror measures in this country," Trudeau said.
Harper echoed that charge in the House of Commons as he dismissed Mulcair's contention that the bill would allow security services to treat those involved in legitimate protest and dissent as would-be terrorists.
"As the NDP's positions on this issue become more and more irrelevant, more and more unconnected to Canadians' real concerns, their statements on this issue become more and more extreme," Harper scoffed.
While he acknowledged that terrorism is a real threat, Mulcair said the government has come up with a "sweeping, dangerous, vague and ineffective" response to it.
The bill, tabled late last month, would give the Canadian Security Intelligence Service more power to thwart suspected terrorist plots.
It would also make it easier for the RCMP to obtain a peace bond to restrict a suspect's movements and extend the amount of time they can be kept in preventative detention.
And it creates a new criminal offence of encouraging someone to carry out a terrorism attack.
"Experts warn that broad measures in this bill could lump legal dissent together with terrorism," Mulcair said. "And the bill would give significant new powers to CSIS without addressing serious deficiencies in oversight."
Mulcair advocates a parliamentary committee with the power to review secret documents like the ones that oversee spies in Britain and the United States.
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