A Senate vote on the government’s plan to legalize pot is expected to move the bill forward Thursday evening in Ottawa but uncertainty abounds over whether travelling and absent senators could allow the Conservatives to snuff out the bill.

Bill C-45, which was introduced in the House of Commons in April 2017, was passed by members of parliament in November 2017 and formed a core part of the Liberal campaign platform in 2015.

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But now, a bill studied and passed by elected members of parliament faces the prospect of potential defeat by unelected senators.

The bill must pass a second reading vote to continue its progress through the Senate and move on to in-depth study by senators at committee, where it has been expected to face fierce questioning.

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It would then face another vote at third reading before moving on to receive royal assent and become law.

If it does not pass, the government would need to introduce a whole new bill and get it through the House of Commons all over again, putting them back at square one just as the clock begins to tick down on what can be passed and implemented before the 2019 election.

Speaking from New Brunswick, Prime Minister Justin Trudeau was asked about the prospect of defeat on Thursday and said that the laws around marijuana need to change to keep money out of the hands of criminals and to regulate who can get their hands on pot.

“I’m confident all Canadians, including senators, understand that,” he said.

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The Liberals and NDP voted to pass the bill through the House of Commons last year, while the Conservatives voted against it.

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While the Liberals hold a majority of seats in the House of Commons and could have passed the bill on their own if needed, the same is not true in the Senate.

Trudeau kicked Liberal senators out of his caucus in 2014 and vowed to appoint only independent senators.

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While 11 senators still call themselves Liberals and generally vote in favour of party legislation, they are not actually part of Trudeau’s caucus and do not have the numbers to guarantee the passage of legislation through the Senate on their own.

Instead, whether the bill passes second reading and continues its progress through the Senate will come down to the new caucus of independent senators who have been either appointed by Trudeau or defected from the Conservative Party.

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Known as the Independent Senators Group, the group of senators appointed since Trudeau took office do not hold party affiliation and vote independently.

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And although there are 43 of them compared to 33 Conservative senators who remain part of that party’s caucus and generally vote in unison, the independents cannot be whipped to vote as a bloc or even to show up for the vote at all.

With roughly a dozen senators travelling at the moment for committee work, it remains to be seen whether enough of the independent senators will come out for the vote or whether enough will vote to support the legislation.

There are also six non-affiliated senators who are neither members of the Independent Senators Group, the Conservative caucus, or the unofficial Liberal caucus.