The federal opposition parties are warning that so-called "right-to-work" legislation that has passed in Michigan could soon come to Canada.

Thousands of people in that state protested Tuesday as the first of two laws designed to weaken union power passed in Michigan's Republican-dominated House of Representatives.

Opponents of the law, including U.S. President Barack Obama, say the law only gives workers the right to work for less pay.

Ottawa-area Conservative MP Pierre Poilievre has already been calling for new rules allowing union members to opt out of paying dues.

The proposal is similar to the legislation adopted in Michigan and 23 other states south of the border — as well as a private members bill proposed earlier this year by Conservative backbencher Russ Heibert.

His financial transparency bill, which was being debated Tuesday in the House of Commons and will likely go to a final vote Wednesday, would require unions to disclose how much money they spend on political activities.

Labour Minister Lisa Raitt said similar laws are already in effect in other parts of the world, including the U.K. and Australia.

"It really is for the workers to have a good understanding how their money is being spent, so they can make informed decisions on how they vote in the people that represent them," Raitt said.

"It's a fundamental part of having rights."

Even if Heibert's bill doesn't pass, the Michigan law is likely to eventually have a ripple effect on other jurisdictions, including Canada, said New Democrat labour critic Alexandre Boulerice.

Interim Liberal leader Bob Rae warned that the Rand formula could be next to come under attack in Canada.

The formula, adopted in the 1940s, makes the payment of trade union dues mandatory in unionized workplaces, regardless of a worker's union status.

Poilievre has said he's heard directly from public servants who are not happy with how one of the country's biggest civil service unions, the Public Service Alliance of Canada, manages its union dues.