SEOUL — A left-leaning politician with deep ties to anti-American activists is taking over as South Korea’s president six months to the day after his arch-enemy tried to muzzle him by imposing martial law. 

The leader of the Minju or Democratic Party, Lee Jae-myung, won revenge against the previous South Korean president, the conservative Yoon Suk-yeol, by capturing 49.3 percent of the 35.2 million votes cast, compared to 41.3 percent for a former labor minister, Kim Moon-soo, in a “snap election” held 60 days after Mr. Yoon’s ouster. 

In a sign of the passions engendered by Mr. Yoon’s abortive martial law decree, nearly 80 percent of the country’s 44 million eligible voters cast ballots — the highest percentage since the leftist Kim Dae-jung was elected in 1997 in the midst of a global financial crisis. Mr. Lee is to be formally inaugurated Wednesday. 

Mr. Lee spoke in exultant generalities as he left his home and then appeared before several thousand fans loudly cheering and shouting his name. Vowing to “do my utmost to fulfill the heavy responsibilities,” he said he was determined not to “fall short of the people’s expectations.” 

The solid endorsement at the polls meant that Mr. Lee would be free to move away from the pro-American policies of Mr. Yoon, who formed tight ties with Washington including revival of large-scale joint military exercises. Mr. Lee has said he will “strengthen” the Korean-American alliance, but he’s also expected to want to improve frayed ties with Communist China and resume dialogue with North Korea. 

He can revise South Korea’s policy while also wreaking vengeance on Mr. Yoon, who narrowly defeated him in the 2022 presidential election and then sought to destroy him by issuing a martial decree on December 3. Defying soldiers who surrounded the national assembly building, Mr. Lee summoned all members of his Minju in the assembly to vote down the decree. 

That was just the beginning of the downfall of Mr. Yoon, who was impeached by the assembly and then ousted by the constitutional court. Mr. Lee as president will have to decide whether to risk deepening the right-left divisions in Korean society by pressing charges of “insurrection” and “abuse of power” in a lower-level court. 

More immediately, Mr. Lee also must deal with a 50 percent rise in tariffs on steel imports as ordered by President Trump. The topic may well come up in a phone conversation with Mr. Trump that aides would like to propose as a prelude to a full-scale in-person summit, probably in Washington. 

He might also see Mr. Trump on the sidelines of the next NATO summit this month in the Hague or at the Group of Seven advanced industrial nations in Canada, also this month, assuming he’s invited as an “observer.”

A big topic in any meeting between the American and South Korean presidents is sure to be the depth of Washington’s historic commitment to South Korea’s defense. 

Mr. Trump and his defense secretary, Pete Hegseth, have proclaimed China as America’s foremost enemy while encouraging South Korea to do far more for its own defense. South Koreans are overwhelmingly in favor of keeping about 28,500 American troops in the country despite reports that Washington is planning to bring some of them back home. 

Reduction in the number of troops in Korea might not conflict with Mr. Lee’s own calls, years ago, for weakening the bond with Washington. Mr. Lee’s Minju includes leftist activists along with left-of-center moderates. Mr. Lee is believed willing to compromise, as did the previous Minju president, Moon Jae-in, who stopped large-scale joint military exercises while appealing in vain for reconciliation with North Korea.

In his rise to power, Mr. Lee, 61, has won a reputation as an unscrupulous political operative involved in corrupt dealings as mayor of a city next to Seoul and as governor of the province that surrounds the capital. He’s also reputed to be a good administrator capable of instituting bureaucratic reform.

By getting elected president, Mr. Lee is avoiding trials on charges of corruption; they may be either postponed or dropped. Opposition politicians are sure to keep harping on that theme, calling for justice, but the humiliation of Mr. Yoon’s failed martial law decree, followed by his ouster, has left his People Power Party divided and confused.