SEOUL, South Korea — The head of South Korean President Yoon Suk Yeol’s governing party on Friday signaled more openness to his removal as president for plunging the key U.S. ally into chaos with a short-lived declaration of martial law, reversing his position the day before an impeachment vote.

Han Dong-hoon, leader of the conservative People Power Party (PPP), had earlier said he opposed impeaching Yoon because it would only create more turmoil. But on Friday he said he had learned…