Malaysian Prime Minister Najib Razak’s fight for political survival has divided the ruling United Malay National Organization and hurt the nation’s economy. Now it is sparking racial discord. Malaysia’s history of ethnic strife, including the 1969 riots in which hundreds of Chinese were killed, makes this development especially troubling.

At the end of August, a series of rallies by the reform movement Bersih demanded that Mr. Najib resign because of corruption allegations that he denies. The main Malay opposition party PAS didn’t take part as it did in past Bersih rallies, so the crowd of at least 50,000 was mostly made up of Chinese and Indian minorities.

That gave Mr. Najib’s supporters a pretext to claim that the main ethnic Chinese opposition party, the Democratic Action Party, is part of a conspiracy to bring down the Prime Minister and take away the affirmative-action privileges reserved for the Malay majority. On Sept. 16 the most radical hotheads, known as red shirts, tried to rally in Kuala Lumpur’s Chinatown, scene of the worst violence in 1969.

To the government’s credit, the police have largely kept the red shirts out of Chinatown and prevented violence. But UMNO politicians lead the red-shirt movement, which is well funded and organized. UMNO-controlled media outlets have joined in the portrayal of opposition reformers as a threat to the Malays. Mr. Najib defended the rally as a response to insults against Malay leaders.

On Friday Chinese Ambassador Huang Huikang waded into the controversy. Speaking at a holiday event in Chinatown as the red shirts planned another foray into the area, he said that the Chinese government opposed terrorism, extremism and discrimination based on race. “But with regard to the infringement on China’s national interests, violations of legal rights and interests of Chinese citizens and businesses, which may damage the friendly relationship between China and the host country, we will not sit by idly,” he warned.