The Senate will vote around 1800 GMT on a "super-amendment" to the bill, which falls short of homosexual rights groups' demands by excluding key provisions.
Renzi has bowed to Catholic pressure to remove stepchild adoption rights from the text to ease its passage through parliament, as well as any reference to the need for faithfulness in such unions.
Italy is the only major European country to have so far given no legal rights to same-sex couples.
The Italian premier's party struck a deal late Wednesday with coalition partners on a revised version of the bill to put to a confidence motion in the Senate.
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While watered down, the text maintains provisions including the obligation to mutual moral and material support, the right to a residence permit for foreign partners and to take a same-sex partner's name.
After the vote on the "super-amendment," the bill could be adopted by the Senate as early as Friday, before being passed by the Chamber of Deputies, where Renzi has a more comfortable majority, in theory getting definitive approval within weeks.
But gay rights groups have voiced anger, on the streets and on social media, saying the bill has been diluted too much.
"We haven't waited 30 years for this," said a joint statement today by some 30 groups, vowing to express their "anger... In the streets, before the courts and at the ballot box."
"Today the Senate is preparing to write a dark page in the history of civil rights in our country by approving a law which... Completely ignores the existence and demands of gay couples' children," they said.