SEOUL, South Korea — President Moon Jae-in’s governing party in South Korea won a landslide in parliamentary elections on Wednesday, as he leveraged his surging popularity over his country’s largely successful battle against the coronavirus to increase his political sway.
With more than 99 percent of the votes counted, Mr. Moon’s left-leaning Democratic Party had won 163 seats in the 300-member National Assembly, according to the National Election Commission on Thursday morning. A satellite party the Democrats created for Wednesday’s elections won 17 seats. Together, the two groups took three-fifths of all seats, giving Mr. Moon the largest majority of seats in three decades.
The main conservative opposition United Future Party and its own satellite Future Korea Party suffered a crushing defeat, winning 103 seats between them. The remaining seats were taken by independents and candidates from smaller parties.
Pandemic or not, South Koreans proved eager to vote in the election, widely seen as a midterm referendum on Mr. Moon, elected to a five-year term in 2017. The voter turnout was 66.2 percent, the highest for a parliamentary election in 28 years.
Wednesday’s election marked the first time in 16 years that left-leaning parties have secured a parliamentary majority, as South Koreans expressed their support for Mr. Moon’s government, which has won plaudits for bringing the epidemic under control.
Their victories could embolden Mr. Moon to reinvigorate his stalled diplomacy with North Korea and press ahead with domestic priorities, like reforming state prosecutors’ offices, which have long been accused of abusing their power.
In South Korea, elections typically have been decided by regional loyalties, ideological differences over North Korea or issues like the economy and corruption.
But this time, “how the government has responded to the coronavirus was the most decisive factor in the president’s approval ratings and in the parliamentary election,” said Park Si-young, head of WinG Korea, a Seoul-based political survey company.
The prospects for Mr. Moon’s party did not look good until less than two months ago. He and his party’s approval ratings had been slumping over a decaying job market, stalled diplomatic efforts with North Korea and scandals involving Mr. Moon’s closest allies. The coronavirus had initially appeared to work against Mr. Moon and his party, as they were criticized for underestimating the threat.
But their political fortune shifted once Mr. Moon’s government began testing large numbers of people in February to screen out patients for isolation and treatment. South Korea, once home to the world’s second-largest outbreak, with as many as 813 new cases a day, has reported fewer than 40 new patients a day in the past week.
As President Trump and other foreign leaders called Mr. Moon, asking South Korea for supplies of test kits or advice in handling the outbreak, his popularity rebounded at just the right time.
During the campaign, Mr. Moon’s conservative rivals accused him of coddling the nuclear-armed North Korea and undermining the alliance with Washington by taking Seoul too close to Beijing, criticisms which Mr. Moon strongly rejected.
With their victories on Wednesday, South Korea’s liberals achieved more political clout than they have ever held.
Their ascent began when former President Park Geun-hye, a conservative, was impeached and then ousted on corruption charges in 2017. Mr. Moon won the presidency in an election that same year, becoming the first left-leaning president in nearly a decade. In 2018, his party won all but three of the 17 contests for big-city mayors and provincial governors.
“By taking over the Parliament as well, the progressives complete replacing the conservatives as the mainstream political force in South Korea,” said Park Sung-min, head of Min Consulting, a political polling company in Seoul.
Anti-North Korea conservatives had dominated politics, the news media and other elite groups in South Korea during the decades following the 1950-53 Korean War. It was not until 1998 that South Korea elected its first left-leaning president, the Nobel Peace Prize laureate Kim Dae-jung. Mr. Kim was succeeded by another liberal, Roh Moo-hyun, president from 2003 to 2008.
But neither Mr. Kim nor Mr. Roh could obtain the type of parliamentary majority that Mr. Moon just earned.
Older South Koreans have tended to vote conservative, criticizing anything less than unequivocal support for the alliance with Washington as “pro-North Korean.”
But postwar generations of South Koreans have more readily seen a need for diplomacy with North Korea and have voiced skepticism over the power of chaebol — family-controlled business conglomerates like Samsung — that have dominated the economy with the help of close, and often corrupt, political ties.
Surprise winners included Thae Yong-ho, the first defector from North Korea to win an election in the South. Mr. Thae, a former North Korean diplomat in London who defected in 2016, won the contest in a Seoul district as a candidate for the conservative United Future.
The polling in South Korea was one of the first national elections taking place amid the coronavirus pandemic, and the country took significant safety precautions to try to ensure that infections did not spread.
All voters were required to wear masks and line up at three-foot intervals. Officials screened out those with high temperatures so that they could vote separately. Voters were also required to rub their hands with sanitizer and put on disposable plastic gloves handed out by officials before entering voting booths.
More than 13,000 South Koreans who were in a mandatory two-week quarantine but still wanted to cast ballots were escorted by officials to vote after the polling stations closed to the general public at 6 p.m. Hospitalized patients of the virus were given the choice to vote by mail. Hundreds of patients with mild symptoms were allowed to vote in advance.
The outbreak also changed the campaign scenes: Candidates replaced handshakes with elbow and fist bumps. Instead of loud singing and dancing, their volunteer helpers handed out name cards and fliers.
The election in South Korea “tells other world leaders that how they respond to their own crisis could make or break their political fortunes,” said Duyeon Kim, a senior adviser on Northeast Asia and nuclear policy at the International Crisis Group. “Because the pandemic is at the top of everyone’s mind.”