The government's cannabis bill faces outright defeat in the Senate today if it can't secure enough support from members of the Red Chamber.

According to an agreed timetable, senators will hold a vote at second reading of the bill today. If the opposition Tories cobble together enough votes to defeat C-45, the legislation would be effectively dead — meaning the government would have to restart the entire legislative process in the House.

If that happens, a summer timeline for legalization becomes much less likely.

Most Independent and Liberal senators generally support the bill, and together they hold 54 of the chamber's 93 occupied seats. But that doesn't guarantee that all of those members will vote together as a block or be present when the vote is recorded.

New senators have been appointed as Independents — leaving the Liberal government with no mechanism to whip votes or force them to attend sittings in the chamber.

Moreover, two Senate committees are on the road: members of the agriculture committee are meeting with stakeholders in Calgary, while the Aboriginal peoples committee is in Winnipeg. That means as many as 20 senators — most of them Independent — will be out of the mix today when a vote is held. Others, like Independent Quebec Sen. Jacques Demers, are not expected to be in attendance because of illness.

The 33 Conservative senators generally vote in lockstep on government legislation, since they all still sit as members of a national party caucus.

Government sources, speaking on background, suggest they have enough support from Independent senators to win the vote. But the slim margin is ringing alarm bells on the government side, as only a few absences could derail a key government policy.

When asked if the government has a plan B ready in the event the bill is defeated, a spokesperson for Justice Minister Jody Wilson-Raybould said the minister still expects the legislation to face a final vote in the Senate on or before June 7, a timeline the Senate leadership agreed to last month.

"The minister has been following the Senate's debate on C-45 since it came to that place last November. She looks forward to appearing before the Senate Standing Committee on Legal and Constitutional Affairs as part of the 'pre-study' of certain aspects of the legislation," the spokesperson said.

Peter Harder, the Liberal government's point-man in the Senate, said Thursday morning he simply wasn't sure he had the votes to get the bill over the line today.

"I am never assured how any vote in the Senate will unfold and, therefore, it would be presumptive of me to predict. What I can tell you is that Conservative senators have said they will be voting against this," he said. "This is a serious vote on a very important bill."