She outperformed her opponent in three days of opinion polling, setting herself up to be named as the Democratic Progressive Party's candidate in the January 2020 election.
Tsai's tough stance against Chinese pressure to reunite with the mainland has driven up once-sagging approval ratings. Her nomination is all but sure to be ratified by the party next week.
She has sought public favor by strengthening the military and cultivating ties with the United States, said Huang Kwei-bo, vice dean of the international affairs college at National Chengchi University.
"Tsai Ing-wen is continuously saying 'I'm going to protect Taiwan and stand up for autonomy and make the most of foreign relations,'" he said.
Her government has helped break ground on a submarine manufacturing site, accepted high-level visits from the United States and received American military support, including seven U.S. Navy ship passages over the past year through the strait dividing China from Taiwan.
China criticizes the high-level visits and the naval ship passages.
Chinese President Xi Jinping said in January that the two sides should be ruled like Hong Kong, which is grappling with mass street protests targeting Chinese control. Hong Kong is part of China under a "one country, two systems" framework that gives the territory some local autonomy.
More than 80% of Taiwanese said in a government survey in January that they prefer self-rule over merging with China.
"What Taiwanese feel most deeply about this is that 'one-country, two systems' is not viable and not acceptable for a democratic Taiwan," Tsai told reporters Thursday afternoon after her secretary general met with Hong Kong students at the presidential office.
The Democratic Progressive Party, which embraces more autonomy for Taiwan instead of a tie-up with China, announced that Tsai had defeated her only challenger, former Premier William Lai, in three days of opinion polling.
Tsai received approval from 35.7% of the roughly 15,000 people polled against Lai's 27.5%, party Chairman Cho Jung-tai said.
In response to her victory, Tsai called later Thursday for "uniting all our power that can be united" to "defend Taiwan."
Tsai will go up against a candidate from the opposition Nationalist Party, which advocates closer relations with Beijing.
Possible Nationalist candidates include Terry Gou, chairman of the world's largest consumer electronics assembler, Foxconn Technology, and Han Kuo-yu, the populist mayor of southern port city Kaohsiung.
Tsai fell below 30% in public opinion polls last year as Taiwanese worried about inaction toward China as well as domestic economic issues such as jobs and housing prices.
Tsai upsets Beijing because she rejects its dialogue condition that both sides belong to a single China, and seeks a closer relationship with the United States. China has reacted by picking off five nations that had diplomatic ties with Taiwan, scaling back Taiwan-bound group tourism and flying military aircraft near the island.
"Looking at the Taiwan election from China's point of view, another term for Tsai would be a disaster," said Denny Roy, senior fellow at the East-West Center in Honolulu.
China has claimed sovereignty over Taiwan since the Chinese civil war, when Chiang Kai-shek's Nationalists lost and rebased their government in Taiwan. The two sides have been ruled separately for seven decades.
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