Labor is taking the rare step of calling on the governor-general to revoke the appointment of unions royal commissioner Dyson Heydon.
A Senate motion on Tuesday comes as unions consider whether to take action to disqualify Mr Heydon over his acceptance of an invitation to speak at a Liberal Party event in Sydney, which he withdrew from last week.
The opposition is seeking crossbench backing for a motion - not used in the upper house for decades - "addressing" the governor-general.
The motion says Mr Heydon has "failed to uphold standards of impartiality expected of a holder of the office of royal commissioner" by accepting the speaking invitation.
Labor frontbencher Penny Wong said the prime minister had declined to act.
"If the prime minister refuses to act in the light of these revelations then it is up to the parliament to act," Senator Wong told reporters in Canberra on Tuesday.
"I don't think any fair-minded Australian believes this commissioner can be seen to be impartial."
The motion, based on advice from the Senate clerk, could come on for debate as early as Wednesday.
Senator Wong said what Governor-General Sir Peter Cosgrove did with the motion was up to him.
Prime Minister Tony Abbott earlier told parliament anyone concerned about Mr Heydon should use "standard procedures".
"They should bring an application in the usual way," Mr Abbott said, adding that Labor had run a "feral" campaign against the former high court judge.
The ACTU has until 2pm on Thursday to lodge its application to have Mr Heydon disqualified and if it does so it will be heard in Sydney on Friday morning.
ACTU secretary Dave Oliver said the union movement is still considering its options.
Asked if it was appropriate for Mr Heydon to decide on his own disqualification, Mr Oliver told ABC radio it was in the first instance and then parties could take the matter elsewhere.
Prominent human rights lawyer Julian Burnside praised Mr Heydon as an honourable man but said he should step aside over the allegations of bias.
"The more his connections with the Liberal party become apparent the more difficult it is to give him the benefit of the doubt but I still think he's an honourable man," Mr Burnside said.
It would be possible to continue the royal commission into trade unions with another commissioner, he said.
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