Mulcair said Bill C-23, dubbed the Fair Elections Act, remains a travesty, even though the Harper government has backed off some of the most contentious provisions.
Indeed, he asserted the bill is aimed at ensuring the Conservatives can cheat during elections without getting caught.
With a final vote in the House of Commons scheduled for late Tuesday, he urged 20 Tory MPs, who he said have shown an interest in promoting and protecting democracy in the past, to show the courage of their convictions.
"Our message to the Conservatives who talk a lot about the importance of our democratic institutions is it's time to walk the talk," Mulcair said.
"It's time to show that you actually do care because this bill still contains several things that are profoundly anti-democratic."
MPs were to spend several hours late Monday voting on amendments proposed by the Tory-dominated committee that studied the bill. They include 45 amendments initiated by the government after the original bill was panned by electoral experts in Canada and abroad.
Only a handful of opposition amendments — all minor housekeeping measures — were accepted by the committee. More than 200 substantive opposition amendments were summarily rejected.
Among the rejected amendments was one that would have given the commissioner of elections the power to compel witness testimony during investigations of suspected electoral wrongdoing. Commissioner Yves Cote has long sought such power, the absence of which has stymied some of his investigations.
After a three-year investigation, Cote announced last month that there was no evidence of a nation-wide conspiracy to prevent non-Conservative voters from casting ballots through automated calls that misdirected voters to the wrong polling stations in 2011. Cote's report said the investigation was hampered by political staffers who refused to co-operate.
The Conservatives' refusal to grant Cote the power to compel witness testimony proves C-23 is aimed at ensuring the ruling party can continue using illegal tactics to suppress non-Tory votes, Mulcair charged.
"You see, what they're holding on to is the ability to continue to stonewall, to obfuscate, to use every trick in the book to hide their voter frauds," he said.
He accused the Tories of having "cheated in every election that they won," starting with the in-and-out scheme in 2006 which allowed the party to exceed its spending limit by more than $1 million. The Conservative party and its fundraising arm were convicted in that case.
In the Commons later, Democratic Reform Minister Pierre Poilievre countered that Cote's robocall investigation found NDP accusations of systematic vote suppression in 2011 to be false. And he maintained the elections commissioner has the same powers as police to conduct an investigation.
After initially calling the bill "terrific" just the way it was, Poilievre last month proposed 45 amendments. Among other things, he backed off on a provision to entirely eliminate the practice of vouching for electors who don't have the proper identification — a measure experts had said could disenfranchise tens of thousands of Canadians.
The amendment would allow those who have ID but none that proves where they live to have their residency vouched for by another elector.
Poilievre yanked entirely a provision that would have exempted money spent on fundraising among previous donors from a party's campaign spending limit. Experts had complained this measure would create a huge loophole that would allow rich, well established parties like the Conservatives to spend untold millions more during campaigns.
Poilievre also relented on a provision that would have effectively muzzled chief electoral officer Marc Mayrand, allowing him to speak publicly only about the mechanics of how, where and when to vote. That was amended to ensure Mayrand can continue to speak freely but to limit Elections Canada's advertising strictly to the nuts and bolts of voting.
A provision requiring automated calling services to retain their records for one year was changed to three years.
However, Poilievre did not relent on other contentious provisions or omissions, such as the proposed elimination of voter information cards as proof of address.
He rejected suggestions that robocall companies be required to supply phone numbers of voters called and the scripts used. And he rejected a long-standing demand of chief electoral officers past and present for the power to audit parties' books to ensure they're complying with political financing rules.
While the amendments have improved the bill, NDP democratic reform critic Craig Scott said it remains "a travesty."
After the Commons vote Tuesday, the bill will be sent to the Conservative-dominated Senate for its approval.
Mulcair, who has been championing abolition of the upper house, held out little hope that the unelected chamber will propose further changes to the bill.
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