In political banners, campaign slogans and everyday conversations, South Koreans used two words to convey the high stakes of this week’s parliamentary election: “Judgment Day.” It was an opportunity to issue a verdict on the first two years of President Yoon Suk Yeol, a leader who has made strides on the global stage but is deeply unpopular and divisive at home.

The results, released on Thursday, were disastrous for Mr. Yoon.

Voters pushed him to the verge of being a lame duck, giving the opposition one of the biggest parliamentary majorities in recent decades. He becomes the first South Korean president in decades to contend with an opposition-controlled Parliament for his entire time in office.

The outcome — and the increasingly polarized South Korean political climate that Mr. Yoon helped intensify — heralded deepening deadlock in a country that is crucial to U.S. efforts to counter China and North Korea. It reduces the odds of Mr. Yoon achieving anything that requires bipartisan support. And it raises the prospect of him leaving office in 2027 with little to show other than redirecting his country’s foreign policy toward expanding military ties with Washington and Tokyo.

Mr. Yoon has been proud of his conservative foreign policy. But to many voters, this election was about domestic woes, such as inflation and signs of democratic backsliding. The opposition successfully framed the vote as a referendum on him.

For months, political analysts, local media and even critics within his own party had been warning Mr. Yoon about his “disconnect” from everyday people and his “hubris” in dealing with the opposition, a national disaster, a prolonged strike by doctors and allegations of corruption involving his wife, Kim Keon Hee.