President Moon Jae-in may have delivered a crippling blow to South Korea’s opposition, but he also turned a prime minister who once served him into a potential rival within a more powerful ruling party.

Lee Nak-yon, 67, not only led Moon’s Democratic Party of Korea last week to the largest parliamentary victory since the end of military-backed rule more than three decades ago, he personally defeated the leader of the conservative opposition. That makes Lee a power broker in his own right and the presumptive frontrunner to succeed Moon when he’s constitutionally required to leave office in 2022.

While Lee proved himself a loyal Moon lieutenant during his record 31-month tenure as prime minister, the journalist-turned-lawmaker reached beyond the president’s left-leaning base to build his own following with independents. Lee was credited with drawing stronger support from his home region in South Korea’s southwest, which jockeys with Moon’s southeast for influence in the ruling party.