Brazil’s top electoral court ruled on Friday that former President Luiz Inácio Lula da Silva, who is serving a 12-year sentence for corruption, cannot run for a third term, eliminating the front-runner in a race many see as a struggle between democracy and due process.

Mr. da Silva, a two-time president who left office in 2011 with a record-high approval rating, has become a polarizing figure. Many supporters remain loyal to the metalworker who rose from the factory floor to the presidency and improved the lives of millions of the country’s poorest, giving him a wide lead in polls ahead of the Oct. 7 vote.