By Angelo Amante
ROME (Reuters) - A senator of Italy's co-ruling 5-Star Movement quit the party on Thursday, further reducing the slim government majority in the upper house of parliament and underlining deep divisions within the anti-establishment group.
Luigi Di Marzio is the fifth senator to abandon 5-Star in just over a month, telling the chamber he would join the growing ranks of unaffiliated lawmakers after falling foul of party leaders.
"They are not interested in anything apart from getting you to push buttons (vote)," Di Marzio said, adding his voice to those who have accused 5-Star leader Luigi Di Maio of centralising too much power to his inner circle.
His move meant the four-party coalition can formally count on 159 seats in the 321-seat Senate, just beneath the threshold for a majority. However the government can rely on the backing of a small number of minority parties and independent senators who are not in the coalition but do not want an early election.
"The government majority is stronger than it might seem," said Stefano Ceccanti, a senator with the ruling centre-left Democratic Party (PD).
"Most parliamentarians do not want an early election. This legislature will carry on until its natural end in 2023."
But the internal haemorrhaging within 5-Star looks set to continue - another senator faces expulsion soon on disciplinary grounds after defying a party requirement to hand over part of his salary to help finance socially useful programmes.
Italy's upper and lower houses have identical powers and a coalition needs a majority in both chambers to govern.
Di Marzio quit after he signed a request for a referendum to overturn a 5-Star initiative to slash the number of parliamentarians by a third at the next election.
The reform was approved in October, but because it involves a change to the constitution, parliamentary opponents have the right to call a referendum to challenge the measure.
The 5-Star obtained 33% of the vote in the 2018 national election but since then its popularity has fallen sharply and latest polls put it around 15%.
(Additional reporting by Crispian Balmer; Editing by Mark Heinrich)