Finally, Tom Mulcair’s New Democrats have moved off the fence.

They say they will vote against Prime Minister Stephen Harper’s sweeping new anti-terror bill — at least in its present form.

That the official Opposition has decided to oppose the government would usually not be news.

But until now the parliamentary debate over Bill C-51 has been oddly timid.

Liberal Leader Justin Trudeau says he has some qualms about a bill that would, among other things, let the Canadian Security Intelligence Service obtain warrants to commit any crime short of murder, assault, obstruction of justice or sexual violation.

However, he and his Liberals plan to vote for it anyway.

That the Liberals are faint-hearted when it comes to supporting civil rights is not surprising.

It was a Liberal government that brought in Canada’s first set of sweeping anti-terror laws.

Moreover, the third-party Liberals still see themselves forming the next government. They know, from their own experience, that draconian security laws can make governing easier.

But the NDP’s initial pussyfooting on this has been more baffling. The New Democrats pride themselves on being a party that stands up for rights such as freedom of speech.

During the 1970 FLQ crisis, 16 New Democrat MPs defied public opinion to oppose the then Liberal government’s invocation of the War Measures Act, a statute that suspended basic civil rights.

Of course, it is always easier for a party with no chance of winning power — like the NDP in 1970 — to take a principled stand.

Mulcair’s NDP, however, has a real shot at becoming the next government. And polls suggest that voters — particularly in Quebec — favour tougher security laws.

So it is perhaps understandable that the NDP’s first reaction to Bill C-51 was less than courageous. (Insiders say Mulcair and his caucus were simply taking time to ponder a complicated bill.)

Whatever the motive, two things happened.

First, Mulcair’s apparent timidity caused consternation among the party’s rank and file. Some questioned his tip-toe approach. They, in turn, were liable to face accusations of disloyalty.

At one point, former party leader Ed Broadbent and former Saskatchewan premier Roy Romanow had to wade into the debate with a very public essay calling on MPs to defeat Bill C-51.

Second, Trudeau’s decision to back the government’s security bill offered Mulcair a new opportunity to present the Liberal leader as an unprincipled lightweight.

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Indeed, once the Liberals decided to support the bill, the NDP had to oppose it. The only question was: How much?

This week, Mulcair used CBC Radio’s Sunday Edition to signal that he would take on Bill C-51 root and branch.

The bill is too sweeping in its scope, he said. The government hasn’t sufficiently explained why such scope is necessary. It puts legitimate critics of government policy, such as pipeline opponents, at risk.

On Tuesday, he told the Commons that the bill would allow CSIS to spy on Harper’s political enemies.

On Wednesday, the formal decision was finalized at the party’s weekly caucus meeting. The NDP would vote against Bill C-51 in its present form.

Echoing the analysis of experts such as University of Ottawa law professor Craig Forcese, Mulcair told reporters the bill’s definitions of crimes against the security of Canada were far too vague.

He pointed out that under current laws, the RCMP and CSIS have been able to break up numerous terror plots. He questioned the need for giving security agencies more powers.

He said giving CSIS the authority to use illegal, dirty tricks against targets was a return to the days when the security services burned down barns.

He said the NDP would use every procedural gambit in its arsenal to slow down passage of Bill C-51.

He acknowledged that this might put his party at odds with current public opinion.

But in the end, he said, the voters would take his side. “Canadians want people who have principles,” the NDP leader said.

Correction-February 20, 2015: This article was edited from a previous version that misstated the name of CBC Radio’s Sunday Edition.

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