Legislative elections take place at the midpoint between presidential elections, and the opposition kept their majority in 2024, so there was hardly a mandate, if anything the legislative elections indicated a rebuke.
Yoon Suk Yeol is the one of the most unpopular democratic heads of state with 19% approval and 72% disapproval.
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It's worth noting that it seems like he didn't talk to anyone about this and even the head of his party in the National Assembly said they would vote to overturn it.