The Senate approved the bill by 184 votes to 66 and it is expected to go before the chamber for a green light in April, Italian media reported.
Once implemented, the revised law should ensure elections result in strong governments with clear parliamentary majorities rather than bickering and shaky coalitions, as has been the pattern in Italy for most of the period since World War II.
The law was forged out of a bitterly contested deal between Prime Minister Matteo Renzi and Silvio Berlusconi and has met stiff resistance among the rebel wing of Renzi's centre-left party, but it is not expected to run into trouble in the lower house, where the government holds a strong majority.
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