In one of its first acts after winning the 2015 federal election, Prime Minister Justin Trudeau's government moved to reverse two Conservative laws that Canada's labour leaders viewed as an attack on unions.

When the House of Commons passed the legislation last fall, Mr. Trudeau boasted of this accomplishment to a large and appreciative gathering of the Canadian Labour Congress.

Now that pledge is suddenly in jeopardy after the Senate voted to keep one of those two laws in place.

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The Senate has amended the government's Bill C-4 in a way that reverses part of its original intent, meaning the House of Commons must now vote on whether to accept or reject the Senate's changes. The government is already signalling it will oppose the Senate amendments, setting the stage for a standoff between the two Houses of Parliament.

The back story of this long-running parliamentary drama dates back to the later years of the Harper Conservative government, when two private members' bills from backbench Conservative MPs – C-377 and C-525 – managed to become law.

C-377, which received royal assent in 2015, required labour organizations to make a series of public financial disclosures, including all transactions of more than $5,000. C-525, which passed in 2014 and came into effect in 2015, forces a secret-ballot vote for any decision to certify or decertify a union.

That replaced the previous practice – known as the card-check system – in which workers could unionize by collecting union membership signatures from a majority of workers. Union leaders said the Conservative change makes it much harder to form a union, because secret-ballot votes tend to be held on the premises of a workplace and the campaigns can lead to intimidation from management.

Critics of the card-check system – including the Fraser Institute – argue a secret ballot protects workers from intimidation from pro-union organizers.

The Trudeau government introduced Bill C-4 in January, 2016. The bill's original intent was to repeal both C-377 and C-525. The House approved the bill in October and sent it to the Senate.

But in a 43 to 34 vote, the Senate voted on Tuesday to amend the Liberal government's Bill C-4 in a way that keeps C-525 and its secret-ballot voting requirements intact. The change was mostly supported by Conservative senators, with the support of some Liberals and independents.

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"The Senate and senators have the duty and responsibility to correct this bill, which was written by the government for the sole purpose of benefiting the powerful union groups that helped it get elected in 2015 in exchange for the measures contained in Bill C-4," Conservative Senator Jean-Guy Dagenais told the Senate in advocating for the amendments.

A spokesperson for federal Labour Minister Patty Hajdu signalled that the government would be voting to reverse the Senate changes.

Senators would then have another chance to vote on the bill when it is returned to them by the House of Commons. Traditionally, the Senate would defer to the will of the elected House, but the Red Chamber has become increasingly unpredictable since Mr. Trudeau began appointed senators who sit as independents.

Hassan Yussuff, President of the Canadian Labour Congress, said he has received assurances from the government that the Senate changes would be rejected.

"For the 60 years that the [card-check] system has been in place, nobody has ever shown any evidence that there were problems with the system that required change," he said. "It prevents employers from intimidating and interfering. … Every time there is a vote, employers do interfere. They express their opinion. They threaten to close the workplace."