The debate has spilled over onto the streets: tens of thousands of supporters demonstrated last weekend throughout the country, while opponents are planning a mass rally in the capital for Saturday.
"Children cannot be bought!" declared a banner torn down and ripped up by police outside the upper house of parliament during a tense stand-off with a group of anti-civil union demonstrators as the debate kicked off inside.
Italy is the last major Western country not to allow gay couples legal status, but after years of thwarted efforts the so-called Cirinna bill, named after the senator behind it, finally landed in the upper house of parliament.
In 2007, a vast demonstration dubbed "Family Day" forced the centre-left government of Romano Prodi to drop a much less ambitious civil union project - and the failure of the bill was cited as one of the reasons behind the fall of his government in early 2008.
But supporters say Italy has no choice this time but to change, the European Court of Human Rights having repeatedly slammed the delay.
The first Senate debate focused on eight potential constitutional issues, to be decided by a vote scheduled for Tuesday, followed by a week of debate on hundreds of amendments and another to review and vote on each article.
'Not a religious issue'
"The law on civil unions cannot become a clash in the streets. Gay people are both right and left wing, as well as Catholics, Muslims, people of every religion, and atheists," said Fabrizio Marrazzo, spokesman for the Gay Centre.
A final vote is expected mid-February, after which the text will have to go before the lower house of parliament.
In its current form, the bill will enable same-sex couples to commit themselves to one another before a state official, to take each other's names and, in certain circumstances, adopt each other's children and inherit residual pension rights.
"It offers the bare minimum. If it doesn't pass as it stands now, then it might as well not pass at all," Marco Simon Puccioni, a film director and gay father to twins with his partner of 11 years, told AFP.
For others, it already goes too far. While centre-left Renzi says he is confident the bill will pass, there are several sticking points, in particular pension rights and the ability to adopt the biological children of one's partner.
Renzi's centre-right coalition allies are categorically opposed to adoption of children by a gay spouse, as is the Catholic fringe of the prime minister's own Democratic Party (PD).
Both camps have threatened to call a referendum if the bill passes as is.
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Meanwhile, left-wing parties in the opposition and the anti-establishment Five Star movement (M5S) - whose support Renzi needs to get the bill through the Senate -- are threatening to pull the plug if even one comma of the text is changed.
Polls show the country fairly equally divided on civil unions, but strongly opposed to adoptions.
The premier, a practising Catholic who opposed the 2007 bill, has said the vote on the article in the bill regarding adoption will be done by secret ballot to allow conscientious objectors free rein.
In the run-up, loud media headlines have been warning the adoption issue could affect 100,000 children in Italy. The country's National Statistics Institute instead puts the figure at between 500 and 1,000 youngsters.
Some have suggested a form of shared parental authority could be implemented while the child in question is a minor, after which it would be up to him or her to say whether they wish to be adopted by their so called "social parent".
But the idea has sent gay rights associations into a frenzy.
And on social networks, a hashtag by activist and transsexual former member of parliament Vladimir Luxuria has been picking up traction against the secret ballot, reading: #mettetecilafaccia or #showyourface.