By Roberto Landucci and Steve Scherer
ROME (Reuters) - Italy's Senate approved an amendment to a new electoral law on Wednesday, paving the way for the passage of a cornerstone of Prime Minister Matteo Renzi's reform program in the upper house next week.
Renzi and center opposition leader Silvio Berlusconi agreed last year to jointly back electoral changes and constitutional reforms to foster more stability and efficiency in long-dysfunctional Italian politics. But the right-left pact has encountered increasing opposition from within both parties.
With votes from both Renzi's center Democratic Party (PD) and Berlusconi's Forza Italia, the amendment containing major elements of the electoral law passed 175 to 110.
Renzi overcame resistance from within his own party, and on Tuesday renewed a pact with Berlusconi, who also shrugged off critics in his bloc, to move ahead with the promised overhaul of the voting measure.
"The fact that this happened with Forza Italia's vote is in my opinion very positive," Renzi told SkyTG24 television at the annual World Economic Summit in Davos, Switzerland.
But Forza Italia dissident Senator Cinzia Bonfrisco said the vote meant "Forza Italia committed suicide". PD lawmaker and former party secretary Guglielmo Epifani described the pact with Berlusconi as "unheard of", saying Renzi "deals with everyone but not with a significant part of his own party".
A final vote on the electoral law in the Senate is expected next week. It will then need to be approved by the Chamber of Deputies before becoming law.
The electoral reform foresees a premium to the party that wins the most votes as long as that group gets more than 40 percent of the total. If no party wins 40 percent in the first round, a second round would ensue, pitting the top two performers against each other. The threshold for any party to get into parliament would be 3 percent.
The previous voting system was widely criticized for forcing parties to join one of two unstable coalitions in order to win, giving small parties inordinate power within governing majorities. That law wrought political deadlock in 2013 when the success of the anti-establishment 5-Star Movement kept any one coalition from winning a majority.
Voting on the electoral changes will be closely watched, with the first round of balloting to elect a new president to replace 89-year-old Giorgio Napolitano starting next week.
Selecting a new president will be a major test for Renzi, who is loath to get bogged down in any stalemate with his reform program still largely incomplete.
(Additional reporting by Gavin Jones; Editing by Mark Heinrich)