Brazil’s Congress on Thursday overturned President Luiz Inacio Lula da Silva’s veto of a law dramatically reducing the prison sentence of his arch-rival Jair Bolsonaro, the second major defeat for Lula in as many days.

Former president Bolsonaro, 71, was sentenced last year to 27 years behind bars for coup plotting over his attempt to cling to power after losing the 2022 elections to Lula.

The left-wing Lula had attempted to block a subsequent push by the conservative-majority Congress to reduce...