CHIAYI, Taiwan—Seeking a second term as Taiwan’s leader, President Tsai Ing-wen has cast herself as a bulwark against authoritarian China’s efforts to undermine democracy in the self-ruled island.

Her main challenger, who advocates restoring closer ties with China, is taking a different tack: criticizing Taiwan’s rambunctious democratic system as toxically partisan and warning about a conspiracy to torpedo his campaign.

Han Kuo-yu, the opposition Nationalist Party candidate for president, trails Ms. Tsai by hefty margins across a range of opinion polls with just weeks left before the Jan. 11 vote—a deficit widely attributed to his perceived leanings toward Beijing, which regards Taiwan as its territory.

In an interview on the campaign trail this week, Mr. Han rejected claims that he is a pro-China candidate and blamed his rivals for creating such a perception. Ms. Tsai and her ruling party “can’t produce a good scorecard so they have to resort to using dual weapons to win the election,” Mr. Han said. “The first is ceaseless smears. … The second is opinion polls.”

To counter prognoses of his potential defeat, Mr. Han offered a response that has surprised many across Taiwan’s political spectrum—that the polls are rigged and that his supporters should either stop responding to pollsters, or actively lie to them about their voting intentions.