Centre Alliance senator Rex Patrick has demanded the removal of a provision giving Mr Porter the ability to apply for deregistration or disqualification.
Commissioner Dyson Heydon's trade union royal commission report recommended only that the Registered Organisation Commission - which has a statutory obligation of neutrality - hold this power.
Mr Patrick said he was concerned that politicians -"particularly Liberal politicians"- may not exercise the power objectively.
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"The other test is that you don't harm the unions that are doing good work for members,"Senator Patrick told theSydney Morning Herald andThe Age.
Mr Porter said in a statement that he was open to making further amendments to the bill,which the government tried unsuccessfully to pass in 2017,in the spirit of"genuine negotiation"with the crossbench.
He said he would only make changes that enabled the bill to"deliver an overall outcome which will achieve the essential necessary result of ensuring registered organisations and officials of registered organisations are required to obey the laws of the land".