OTTAWA -- Conservative senators are leading a charge to gut legislation aimed at cracking down on impaired driving -- voting to delete a measure Conservatives have previously championed.

The Senate's legal and constitutional affairs committee voted late Wednesday to delete a provision from Bill C-46 that would authorize police to conduct random roadside breathalyzer tests, without needing reasonable grounds to suspect the driver may be impaired by alcohol.

The move was proposed by Conservative Sen. Denise Batters on the grounds that the provision is likely to violate the charter of rights and would, therefore, be struck down by the courts as unconstitutional.

She won the backing of four other Conservatives senators on the committee, as well as the committee chair, Liberal independent Sen. Serge Joyal, a constitutional expert in his own right.

Five independent senators voted against deleting the provision, including Sen. Marc Gold, who is also a constitutional law expert. One Liberal independent abstained.

Among the Conservatives who supported deletion was Sen. Jean-Guy Dagenais, a former police officer. Just two years ago, Dagenais joined former Conservative public safety minister Steven Blaney at a news conference, where the MP introduced a private member's bill that contained a similar provision on random alcohol breath tests.

Blaney's bill was applauded by Mothers Against Drunk Driving and received unanimous support in principle among all parties in the House of Commons, with Conservatives being particularly enthusiastic.

So it came as a surprise to many when Conservative senators spearheaded the move to remove the random breath testing measure from Bill C-46.

"It's unacceptable. It guts the bill," Sen. Peter Harder, the government's representative in the Senate, said Thursday.

"It appears as though obstruction of the policy agenda of the government of Canada is of higher priority than consistency of policy position," he added.

C-46 is a companion bill to government legislation to legalize recreational marijuana, which the Conservatives have attempted to block at every turn. However, the provision on random roadside testing is meant to apply only to alcohol.

"This is shocking," said MADD CEO Andrew Murie.

"You kind of wonder if this is based on what they say is legal concerns or is this politics being played out?" he added, accusing Conservative senators of using C-46 to try to delay passage of C-45, the cannabis legalization bill.

The committee's report on C-46, along with its recommended amendments, is expected to be debated in the Senate next week. Harder said he'll move to have the deleted provision restored.

Should the Senate as a whole agree with the committee, Justice Minister Jody Wilson-Raybould made it clear Thursday that the government will not accept an amendment that drops the random breath testing measure.

"I think it's irresponsible," Wilson-Raybould said, adding that the government is "determined to ensure" the provision becomes law.

"Mandatory alcohol screening is the centrepiece of this legislation ... Partisan politics has nothing to do with saving people's lives and mandatory alcohol screening will save lives."

In proposing the amendment to delete the provision, Sen. Batters argued that "defence lawyers with lengthy careers in this field tell us that these provisions would lead to a decade of charter challenges. A "glut" of impaired driving cases has already been a major cause of delays in Canada's criminal court system, resulting in some cases being dismissed, and Batters contended authorizing police to conduct random, roadside breath tests would only exacerbate the problem.

"It's our duty to provide sober second thought, particularly on these types of constitutional issues," she said.

"I know we don't want to see many more serious criminal charges like murder, sexual assault, serious child abuse cases dismissed because of the massive amount of charter challenges brought because of the random alcohol testing provisions in this bill."

However, Gold said he believes the provision is justified under the charter in the interests of reducing the harm caused by drunk driving. He noted that eminent constitutional scholars -- including Canada's pre-eminent constitutional authority, Peter Hogg -- support that contention.

"Mandatory testing will survive court challenge, though challenges there will be, as there have been every time we change the law," Gold predicted.

Wilson-Raybould said she's confident the provision is constitutional.

More than 40 other countries have authorized police to conduct mandatory alcohol breath tests and Liberal MP Bill Blair, the government's point man on cannabis legalization and a former Toronto police chief, said they've found the measure has reduced the number of deaths due to impaired driving by up to 36 per cent. That's evidence the Conservatives once found compelling, he added.

"It's a little bit of a mystery as to why they now reject those arguments that they so passionately advocated for in the past," Blair said.