SEOUL, South Korea — The knife attack last week on the American ambassador to South Korea, Mark W. Lippert, set off an outpouring of good wishes here for both the envoy and Seoul’s alliance with Washington.

But the response, led largely by conservative South Koreans, has provoked a backlash, with accusations that the government of President Park Geun-hye and its supporters are “worshiping” America and politicizing the case to discredit their critics.

Kim Ki-jong, a professed nationalist with a history of erratic outbursts of violence, slashed Mr. Lippert with a kitchen knife during a breakfast meeting last Thursday. The assault left a four-inch gash on Mr. Lippert’s right cheek and damaged tendons and nerves in his left hand.

When the South Korean news media carried images of Mr. Lippert splattered with blood, the public initially reacted with shock. Well-wishers flooded Mr. Lippert’s blog and Twitter account, and they posted messages on signs that conservative activists put up near the United States Embassy in Seoul. The tone of the messages quickly turned into one of guilt and apology.