A spokeswoman for Rep. Robin Vos, the co-chairman of the budget committee, said the revisions would be offered later Wednesday, the same day that thousands of people have descended on the state Capitol to protest the legislation.
Democratic Sen. Lena Taylor of Milwaukee said on her Facebook page that the amendment would be substantive, technical and complex.
No details were immediately available.
Gov. Scott Walker said earlier that he was open to changes in the bill but would not reconsider the fundamental principles of his plan, which is the nation's most aggressive anti-union proposal.