On the campaign trail, Ms. Tsai and other party candidates have denounced China’s interference and warned that under China’s terms for unification, Taiwan could end up like Hong Kong. There, protesters have for months been pushing back against Beijing’s efforts to chip away at the semiautonomous Chinese territory’s unique social and political freedoms.

“With Taiwan’s ballots we want to tell the world that we don’t want to be like Hong Kong,” Taiwan’s premier under Ms. Tsai, Su Tseng-chang, said at a campaign rally in New Taipei City this week. He warned that China’s terms for unification would mean surrendering Taiwan’s sovereignty.

“We want to be the masters of our own country,” he thundered, drawing rousing cheers from a crowd of several thousand Tsai supporters.

Mr. Xiang and his wife, who were in Taiwan when the appeal leaked, were detained by the authorities at the main international airport on Nov. 24 and ordered not to leave the island. It is not clear if they are cooperating with investigators. They could not be reached for comment.