ROME, Sept 15 (Reuters) - A contested reform of Italy's electoral law won Senate approval on Tuesday amid fierce opposition criticism, as rivals accused Prime Minister Giorgia Meloni of changing the rules to improve her chances of winning re-election next year. Meloni's conservative coalition, including Brothers of Italy, the League and Forza Italia, has put forward a fully proportional voting system that would guarantee a parliamentary majority to any alliance winning more than 42% of the vote. The measure still needs final approval from Italy's lower house. Under the current system, most lawmakers are elected through proportional representation, while a third are chosen in first-past-the-post constituencies that analysts say may favour the centre-left opposition. The reform would abolish these. The coalition said the reform, approved by 113 votes to 71, would ensure political stability in a country notorious for short-lived governments. Meloni took office in 2022 and this month became the leader of Italy's longest-lasting administration since World War Two. "This isn't about stability. You're doing it because you believe that there is a risk you will lose the next election," said former prime minister Matteo Renzi, leader of the Italia Viva party. After the vote, centre-left senators waved red and yellow placards bearing the slogan: "No to the fraud law." The reform's initial approval in the lower house in July was overshadowed by a setback for Meloni. Her proposal to reintroduce preferential voting — which allows voters to pick their preferred candidates from party lists — was narrowly defeated in a secret ballot due to defections within the ruling coalition. That vote exposed strains within the alliance ahead of an election due next year, fuelling perceptions that Meloni's grip on the coalition may be weakening at a time when the centre-left holds a narrow lead in the polls. Rather than drop the measure, Meloni, who has long supported preferential voting, brought it back to the Senate, which approved it. Because the reform was amended, it must now return to the Chamber of Deputies, or lower house, for a final vote. Some politicians suggest the new law could benefit Meloni's opponents, who could surpass the 42% threshold and secure the majority bonus, while a new far-right party, National Future, steadily erodes support for her conservative coalition. Meloni has rejected the suggestion she might make an electoral pact with National Future, but polls say such a tie-up would give them a clear lead over the centre-left bloc. The reform would also require each alliance to name its candidate for prime minister before the election, a provision that is deepening tensions among the leaders of the fractious opposition parties. "That could prove problematic for the centre-left, potentially forcing an early leadership contest and exposing divisions that could weaken the bloc," said Rado Fonda, of pollster SWG.