A former premier and vocal proponent of independence for Taiwan will challenge his former boss, President Tsai Ing-wen, for the ruling party's nomination, ensuring the island's tense relationship with China will be a central focus of the presidential campaign.

Former Premier Lai Ching-te said in a text message Monday that he would vie for the Democratic Progressive Party's backing in the election slated for next year. Lai — a former mayor of the southern Taiwanese city of Tainan who once described himself as an "independence worker" — resigned in January, weeks after the party suffered a resounding loss in local elections.

"Taiwan is facing a more precarious situation in the face of China's threats," Lai said Monday, after submitting his nomination application at DPP headquarters in Taipei. "Taiwan doesn't want to be a second Hong Kong or Tibet. The elections in 2020 are critical to Taiwan's future."