Justin Trudeau's pledge to "repeal" the parts of the government's new anti-terrorism legislation doesn't mean unravelling all those new police and spy powers – he says he'd repeal, but he's proposed tinkering.

And Bill Blair, the former Toronto police chief who has joined the Liberals as a star candidate, offers a glimpse of the Liberal balance: He said he supports the new powers for police and spy agencies, and doesn't think they go too far – they just need to be balanced with more oversight and scrutiny, including bi-partisan parliamentary review.

"I think the powers are necessary. I've worked with the RCMP and I've worked with CSIS. I think that the job that they do on behalf of Canadians is critically important to our safety. And I know from experience the threat is real," Mr. Blair, now the Liberal candidate in Scarborough Southwest, said in an interview with The Globe and Mail.

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"At the same time, I also know the expectation of Canadians to ensure that our rights and freedoms are protected. And they have every right to be assured that there is effective – not partisan, but bi-partisan – and independent oversight."

Most Canadians believe more oversight is necessary, according to several opinion polls. But vocal opponents of Bill C-51 have also called for repeal of some of the new police and spy powers, like easier-to-obtain "peace bonds," longer terms for preventive arrest without charge, and the new mandate for the Canadian Security Intelligence Service to use unspecified measures to disrupt threats – including the ability to obtain warrants that allow agents to break the law or violate someone's Charter rights.

Those opponents have beaten up Mr. Trudeau's party for voting for Bill C-51, yet promising to change it if they take power – and the NDP have used it against their opposition rivals.

Mr. Trudeau is now talking about repeal, telling reporters Tuesday his Liberals are "committed to bringing in reforms and repealing the sections of the Anti-Terrorism Act that cause so much concern for so many Canadians." But in reality, apart from adding oversight, the Liberals have been proposing modest amendments.

The Liberals have pointed to amendments proposed by Liberal MP Wayne Easter, but they do not amount to broad repeals – they include limiting the sharing of private information for national security to 17 departments, and instead of repealing CSIS's broad new mandate, removing the provision that allows CSIS to obtain a warrant to violate the Charter.

Mr. Blair said he believes some new powers in the bill are necessary tools, like easier-to-obtain peace bonds, which restrict terrorist suspects to bail-like conditions, and week-long periods of preventive arrest. In fact, he said he does not think any of the powers and mandates in the bill go too far – they just need expanded oversight.

"All cops get this: Society gives us extraordinary powers. Powers to detain people, to lay charges, and to even use force and deadly force. And in exchange for those authorities that Canadians give to their law enforcement officers, we agree to submit to their oversight," he said. "And I believe some of that is missing in Bill C-51."

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Mr. Trudeau, however, now has to soothe critics on the left and right. The NDP's Thomas Mulcair, who says he would repeal the whole bill, has scarred the Liberals with attacks on the issue.

But Mr. Blair's candidacy is itself a good reminder of why the Liberals are trying to split the issue down the middle: they are trying to blunt the Conservative accusation that they're soft on security.

The Liberals hope Mr. Blair, Toronto Police Service chief until two months ago, will help bolster their security credentials – and help dent the Tories.

"They've presented themselves as the only people that are tough on crime. They've been tough-talking on crime, but that doesn't mean smart on crime, and it doesn't have the effect," Mr. Blair said.

But he's also a candidate that might draw some fire. As chief, he was accused of leading a heavy-handed police response to G20 protests in 2010 – criticism he rebuts by noting the police actions were subjected to independent review in a report that was made public. As a candidate, he might help the Liberals defend against Tory attacks, but raise criticism on the left.

And that's the balance the Liberals still find themselves trying to strike on C-51 – as Mr. Trudeau suggests he'd make big changes by talking about repeal, but hasn't yet proposed scaling back most of the new powers.