The Sunflower Movement of 2014, a series of protests led by a coalition of students and civil-society activists, marked the rejection of close relations with China by Taiwan’s younger generations. So did the election of the pro-sovereignty Ms. Tsai in 2016.

Ms. Tsai’s popularity then slid — mostly because she couldn’t sell significant reforms on pensions and same-sex marriage or make progress on stagnant wage growth and pollution control. By the time local elections were held in late 2018, her chances at a second presidential term seemed to be next to nil. But now she leads opinion polls.

For her renewed popularity, she can thank, in part, the monthslong protests in Hong Kong. Beijing designed the “one country, two systems” model in place in the city also with Taiwan in mind. The idea, long unpopular with many Taiwanese people, seems less credible than ever.

China casts a wide net, and it will persist in pulling its military and economic levers. No doubt, too, it will continue to manipulate news coverage to try to buoy Beijing-friendly candidates in the upcoming election. But now it is also launching a disinformation campaign to sap Taiwanese’s trust in their institutions and sow discontent among them.

Late last month, Ms. Tsai accused China of “producing fake news and disseminating rumors to deceive and mislead Taiwanese” in hopes of “destroying our democracy.” Ms. Tsai herself has struggled to shake off the accusation that she did not earn a doctorate from the London School of Economics, even though the university has confirmed that she was “correctly awarded a Ph.D. in law in 1984.” Chinese officials are reported to have privately admitted that Russia’s tampering with the United States’ presidential election in 2016 caused them to reconsider ways of meddling with Taiwan’s.

China has also made no secret of its intention to exacerbate social rifts in Taiwan. An editorial from April in The Global Times, a Chinese state-owned tabloid, stated: “We don’t need a real war to resolve the Taiwan question. The mainland can adopt various measures to make Taiwan ruled by the Democratic Progressive Party (DPP) turn into a Lebanon situation which ‘Taiwan independence’ forces cannot afford.” Meaning: The Chinese government believes it can pit various ethnic, political and social groups in Taiwan against one another.

China can also be expected to exploit the soft underbelly of Taiwanese politics: patronage networks. Those are less important today than during Taiwan’s authoritarian days, but they continue to allow community leaders, farmers’ associations and even organized-crime figures to buy votes.