Labor's push to reverse cuts to Sunday penalty rates appears to be doomed after Nationals MP George Christensen rebuffed an opposition challenge to cross the floor.
A bill reversing the industrial umpire's decision cleared the Senate on Monday.
The Greens and several crossbench senators supported Labor's amendments to legislation scrapping four-yearly reviews of modern awards.
Labor frontbencher Doug Cameron said there was "no excuse" for Mr Christensen to abandon his support for penalty rates.
In July, the maverick Queensland MP introduced a private bill to wind back the Fair Work Commission's decision to cut penalty rates for hospitality, retail, pharmacy and fast-food workers.
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But Mr Christensen won't back Labor's amendments when the legislation enters the lower house.
"Amendment from the Senate does not guarantee no back-pay costs for small business. I can't just go on Dougie Cameron's say so," Mr Christensen tweeted on Monday.
"As such, I can't support the amendment as it is."
Labor's move - labelled a stunt by Employment Minister Michaelia Cash - forced the government in the upper house to oppose its own bill.
With Mr Christensen's support, the government has the numbers in the lower house to defeat the penalty rates reversal.
Senator Cameron says 700,000 workers have been affected by the commission's decision since it took effect on July 1.
"This is exactly why people see the coalition as not being fit for government," Senator Cameron said.
"They do not support penalty rates for workers. They want to attack working people in this country every opportunity they get."
Senator Cash accused Labor of holding otherwise non-controversial legislation hostage to another stunt on penalty rates.
"They would put political point-scoring ahead of the interests of employers, unions and everyday workers," Senator Cash said.
She says the original bill, which also allows the Fair Work Commission to overlook minor and technical errors when approving industrial agreements, made common-sense changes.
Mr Christensen has broken ranks with the government this year over establishing a banking royal commission and restoring penalty rates.
He has admitted he was the anonymous MP last week threatening to quit the party if Malcolm Turnbull continued as prime minister.
