The Conservative-dominated Senate is preparing a rare show of independence as it appears likely to defeat gambling legislation that was supported unanimously by all parties in the House of Commons.

After intense last-minute lobbying by professional sports leagues – including the National Hockey League and Major League Baseball – Conservative and Liberal senators told The Globe and Mail Thursday that a large number from both parties are ready to defeat a private member's bill that would allow Canadians to bet on a single sports event.

The Senate has only defeated a bill eight times in the last 70 years and 133 times since 1867. The fact that this bill was passed unanimously by the House makes this situation particularly unusual.

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The House of Commons approved the bill unanimously in March without a standing vote. It was put forward by NDP MP Joe Comartin, who argued that the current law against betting on a single game diverts millions in potential revenue to organized crime.

Mr. Comartin has said the legal change would bring new revenue to the provinces for social programs.

The NDP MP represents a riding in Windsor, Ont., which has one of the biggest casinos in the country, and Mr. Comartin did not hide the fact that the bill would likely create more casino jobs. Federal Justice Minister Rob Nicholson – who represents a casino-dependent riding in Niagara Falls – has indicated the Conservative government supports the bill.

Provincial governments, including British Columbia and Ontario, have argued in favour of the change. But the federal government's support isn't enough to sway several Conservative senators.

"There's certainly a real effort to defeat the bill," said Conservative Senator Bob Runciman, who is sponsoring the legislation in the Senate but describes himself as an "unenthusiastic" supporter.

"A number of senators in the Conservative caucus are adamantly opposed to the bill."

The former Ontario cabinet minister said there are several reasons why senators oppose the bill – including personal feelings about gambling and concern about the lack of debate the bill received in the House. But Mr. Runciman suggested there are other factors motivating the resistance.

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"Some people see this as an opportunity to send a message that we're alive and well," he said. "It's going to be an interesting period of time dealing with this bill."

The Senate legal and constitutional affairs committee wrapped up its hearings on the bill Thursday morning, meaning it will face a final debate and vote in the Senate when Parliament returns after a one-week recess.

The committee received a statement Thursday from NHL deputy commissioner Bill Daly, who delivered a grim warning of the consequences of single-game betting. The statement says the practice, if allowed, would lead to concerns that NHL games are not legitimate and raise allegations that teams are shaving points and manipulating injuries.

Two Conservative senators on the committee, Linda Frum and Vern White, told The Globe they will be voting against the bill based on the concerns they heard from witnesses, including the representatives of professional and amateur sports.

"I guess we won't really know [the outcome] until the vote happens. I know that I'm certainly not alone," Ms. Frum said. "There's a number of us who feel the same way about this, but I haven't done the count."

Liberal Senator George Baker, who also sits on the committee, says the bill's demise is a done deal. "It'll be defeated," he said.