SEOUL—The elderly man who stabbed South Korea’s opposition leader last week did so wanting to prevent the progressive politician from winning the country’s next presidential election in three years, police said.

Lee Jae-myung, head of South Korea’s Democratic Party, survived the Jan. 2 attack in the southern port city of Busan. He required emergency surgery for a laceration to a major vein of his neck. The alleged attacker, born in 1957 and surnamed Kim, had presented himself as a Lee supporter to draw close to the politician in a crowd of journalists and party officials, police said.