SEOUL, South Korea — There were 28 cases of the coronavirus in South Korea on Feb. 13. Four days had passed without a new confirmed infection. President Moon Jae-in predicted that the outbreak would “disappear before long,” while the prime minister assured people that it was OK not to wear surgical masks outdoors.

As it turns out, the virus had been rapidly spreading at the time through a large, ​secretive ​church in Daegu, where it has since mushroomed into the largest epidemic of the coronavirus outside China, with 2,022 cases, including 13 deaths.

[Read: ‘Proselytizing robots’: Inside South Korean church at outbreak’s center.]

Now the president is facing a political backlash over his response as the number of cases continues to climb — 505 new infections on Thursday alone.​

Opposition politicians are seizing on what they call ​Mr. Moon’s​ ​mishandling of the crisis, by not moving quickly to close the country’s borders to China and not supplying enough surgical masks for citizens. The virus is also intensifying existing pressure from the weak economy, which is being made worse by a sharp decline in trade with China, South Korea’s biggest trading partner.