The Italian Senate gave final approval to electoral reforms on Tuesday but almost two dozen members of Prime Minister Matteo Renzi’s party abstained in protest, angered by the deal he did with right-wing leader Silvio Berlusconi.

The bill, which will go into effect once the lower house approves it as well, would guarantee a governing majority for the party that wins the most votes in elections.

If a party gets at least 40 percent support, it would win a majority of the seats in the lower chamber. If no party crosses the 40-percent threshold, a runoff would be held between the two most popular parties.

A separate reform, enacted last year, reduces the Senate to a body of regional deputies without the ability to block legislation.

The two chambers currently have equal powers. Renzi’s leftist Democratic Party commands an absolute majority in the lower house but not in the Senate where small centrist and conservative parties hold the balance of power.

An earlier version of the law gave a governing majority to the winning coalition. The change to winning party has fueled suspicions on the left. Berlusconi still backed the reforms even though his Forza Italia hasn’t polled above 15 percent support since last year. Only in coalition with the separatist Lega Nord would it have a shot at winning 40 percent support nationwide.

“What’s in it for Berlusconi?” Democratic Party deputy Giuseppe Civati and other party rebels wondered.

Renzi said there was no hidden agenda. But by working with the prime minister, Berlusconi, who last served as premier between 2008 and 2011, has secured a central role for himself in talks over who will succeed Giorgio Napolitano as president of the republic.

Napolitano resigned earlier this month. There is no clear frontrunner to replace him.

The president can nominate prime ministers, dissolve parliament and call elections. He also has the power to grant pardons, something Berlusconi, the subject of many scandals and investigations, might be especially interested in.

Berlusconi and Renzi agreed last year to jointly enact constitutional changes.

Their parties were forced into a coalition after the anti-establishment Five Star Movement prevented either from winning a majority in 2013.

Berlusconi later quit the coalition but the government was saved when centrist rightwingers under Angelino Alfano, the interior minister, split from Forza Italia and established their own party, Nuovo Centrodestra. It is polling at around 3 percent, barely enough to cross the electoral threshold introduced under the new voting law.