BRASILIA, Oct 6 (Reuters) - Two right-wing political parties in Brazil decided on Wednesday to join forces to become the country"s largest political party, with plans to field an alternative to President Jair Bolsonaro in next year"s elections. The Democratas (DEM) and the Social Liberal Party (PSL), which Bolsonaro joined ahead of his 2018 campaign, will have a combined 82 seats in the 513-member lower chamber of Congress, a larger grouping than the left-wing Workers Party (PT) with 53. The merger must still be approved by Brazil"s top electoral authority, but the new party, called Uniao Brasil, could shake up a 2022 presidential race that now looks polarized between the far-right Bolsonaro and former leftist President Luis Inacio Lula da Silva. Lula, who founded the Workers Party and served as president from 2003-2010, holds a commanding double-digit lead in opinion polls over Bolsonaro, whose popularity has plummeted over his handling of the world"s second-deadliest COVID-19 outbreak, high unemployment and rising inflation. The Uniao Brasil party will be headed by Luciano Bivar, head of the PSL, which grew from nowhere to become the largest in the lower house on the conservative wave that elected Bolsonaro to the presidency. Bolsonaro quit the PSL in 2019 in a quarrel over control of the party and its growing campaign coffers. DEM leader Antonio Carlos Magalhaes Neto, a former mayor of Salvador and rising star from a powerful family in Bahia state, will be the new party"s secretary general. "This party is born with the mission to recover optimism in Brazil and restore the trust of Brazilians in democracy," he said in a convention speech announcing the fusion. He told reporters he expects the new party to field its own presidential candidate next year, backed by a renewed right-wing coalition challenging Bolsonaro, who has no party at the moment. Reporting by Anthony Boadle; editing by Grant McCool