Justin Trudeau cut ties to the Liberals in the Senate last year, declaring them free to act as independents. Now they are flexing that muscle.

Many, if not all, of the members of the Senate Liberal caucus will vote against the Conservative government's controversial anti-terrorism bill, C-51, which Mr. Trudeau and the other Liberal MPs supported last week when it was passed in the House of Commons.

(For more on Bill C-51, read The Globe and Mail's in-depth explainer: Privacy, security and terrorism: Taking a closer look at Bill C-51)

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"We are not part of their caucus," James Cowan, the leader of the Liberal opposition in the Senate, said Tuesday. "Mr. Trudeau speaks for the Liberal Party of Canada. We don't speak for the Liberal Party of Canada. We don't speak for the Liberal caucus. We are senators, we are independent, and we make our own decisions."

Many traditional Liberal supporters were angry when Mr. Trudeau and Liberal MPs voted in favour of the Conservative proposals for tackling terrorism. Some people, who argued the bill will violate civil liberties, posted on Twitter with photos of Liberal membership cards being cut in half.

And the Liberal Leader, himself, was not wholly in favour of the bill, which would give police new powers of preventative arrest, provide spy agencies with new abilities to disrupt threats, broaden the scope of the no-fly list, ease the transfer of information between some federal agencies, and criminalize the promotion of a terrorist attack.

But the Liberals did not want to provide the Conservatives with election campaign opportunities to label them as being soft on terrorists. Mr. Trudeau told a student audience in Vancouver earlier this year that he did not want to give the Conservatives the chance to make "political hay" out of the security issue. "This government would be perfectly happy if the opposition completely voted against this bill because it fits into their fear narrative," he explained.

So, after the Conservatives voted down some proposed Liberal amendments, the Liberal MPs plugged their noses and voted for Bill C-51.

Now Mr. Trudeau says he will amend the legislation if his party forms government after the country goes to the polls in the fall.

But Mr. Cowan says he and his fellow Liberal senators don't like the bill and they are under no obligation to support it.

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"I think they've got the balance wrong," he said. "I recognize the dangers of international terrorism and domestic terrorism and we need obviously to be looking at the powers that our police and intelligence services have and making adjustments. But, in our democracy, we always talk about balancing those powers against civil liberties and human rights and my sense is that this has gone too far."

Because the Conservatives have the majority in the Red Chamber, there is almost no chance that the bill will be defeated even without the backing of Liberal senators. But, opposing C-51 is "a statement," said Mr. Cowan. "There are some serious concerns about the constitutionality of this bill and we're putting those on the record."

Of particular worry, he said, is the government's refusal to incorporate Parliamentary oversight on police and intelligence authorities like that which exists in the other "Five Eyes" countries that are Canada's security partners – the United States, Great Britain, Australia and New Zealand.

Mr. Cowan said he has had no discussions with Mr. Trudeau or his office about the intention of Liberal senators to oppose the bill.

"So nobody can blame him for our decisions, or us for his decision," he said. "I am sure if the Conservatives decide they are going to tar Mr. Trudeau with our brush, then they can do that. But it's not right and if he wants to be critical of the stand we take, he's entitled to do that."

Mr. Cowan recalled the the early years of the Conservative government when Prime Minister Stephen Harper and his government were introducing measures aimed at imposing harsher punishments on criminals. Some of those bills were difficult for Liberal senators to support, he said, but there was always a fear that the Conservatives would accuse the Liberals of being soft on crime if their caucus members in the Senate voted against them.

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Many times, said Mr. Cowan, Liberal senators have been forced to say: "'Maybe we will just let this one go.' Well, we don't have to do that any more."