Italy is nearing a landmark vote in favour of same-sex civil unions but the victory is looking increasingly hollow to gay rights supporters after the bill was dramatically watered down to win the backing of conservative and Catholic senators.

The intensity of the political fight over the legislation has shown the enduring influence of the Catholic church on Italian politics, and the willingness of opponents of the prime minister, Matteo Renzi, to exploit the issue to score political points.

In order to get the bill into a position where it could be passed, Renzi was nevertheless forced to make a major concession on the parental rights of same-sex couples, despite this week rejecting the idea that Italy could have “Serie A and Serie B children” – a reference to the Italian football league.

The bill is expected to be voted on by the senate on Thursday. It will then be voted on in the lower house, where Renzi has more support.

An amendment known as the “stepchild” provision that would have granted parental rights to a non-biological parent in a same-sex union was stripped out of the legislation this week.

The decision came days after the populist Five Star Movement, Italy’s second largest political party, backed out of an agreement to work with Renzi’s Democratic party to overcome parliamentary hurdles against the proposed legislation.

The decision was probably a pragmatic one by the M5S leader, Beppe Grillo, who analysts said was looking to make in-roads with conservative voters ahead of local elections this summer.

Renzi was then forced to turn to his conservative coalition partners in the New Centre Right to get the legislation passed.

But after winning the major “stepchild” concession, members of the New Centre Right party were on Wednesday pushing for more compromises.

“Scrapping stepchild adoption is not sufficient,” said Beatrice Lorenzin, the Italian health minister. “At the moment the [ruling] Democratic party and my party are working to build an amendment [to the bill] to avoid [civil unions] being given the same status as marriage, which we consider unconstitutional.”

Lorenzin has been a leading voice against the provision granting parental rights and has argued that its passage would see a big increase in same-sex couples seeking to have children through surrogacy, which is illegal in Italy. She has called surrogate motherhood “ultra prostitution” and said all attempts to legitimise surrogacy and the donation of sperm and eggs to allow same-sex couples to have children had to be “denounced without hypocrisy”.

Gianni Riotta, a journalist and professor at Princeton University, said the fight over the civil unions bill had exposed a fact that sometimes gets overlooked in Italian politics: that the parliament is deeply divided.

“Renzi being a magician has managed so far to hide this. but when things got back to basics, it was clear this was the reality,” Riotta said.

While the bill will be far from perfect, Renzi will have managed to achieve what his predecessors did not.

This could ultimately reflect not just Renzi’s political manoeuvres, but the fact that another important change has occurred in Italy since the last time such legislation was tried and failed. Pope Francis, who was elected in 2013, has publicly stated his desire to stay out of Italian politics, despite his concern over gay unions.

While the Catholic church has certainly been influential in the political battle, including vocal opposition by members of the Italian bishops conference, it has not killed the bill.

Riotta said it was likely same-sex couples would eventually win parental rights, but that they are likely to win the concessions in court, not parliament.

“This is the Italian way of kicking the can down the road and doing tomorrow what you could have done yesterday,” he said.