South Korea's opposition party on Thursday filed an impeachment motion against Acting President Han Duck-soo.

The motion was filed after President Han refused to appoint judges to the Constitutional Court, a step required to complete the process of removing his predecessor from office.

The country has been in a political crisis since suspended President Yoon Suk-yeol declared martial law on December 3. On December 14, the National Assembly stripped Yoon of his powers, but the Constitutional Court's ruling supporting the parliamentary decision is required to complete the impeachment process.

However, Acting President Han blocked the process by refusing to approve the appointment of three nominated judges to the nine-member Constitutional Court bench, effectively halting the procedure to remove Yoon from office. As a result, the opposition Democratic Party is now seeking to impeach Han as well.

"We have submitted the motion... and will report it in the full session today," said lawmaker Park Seung-jun, speaking to reporters in the National Assembly regarding the action against Han. "We will put it to a vote tomorrow."

"Han's refusal to formally appoint the three judges proves that he lacks the will and ability to uphold the Constitution," said Park Chan-dae, the Democratic Party’s floor leader.

If the court proceeds with impeachment without appointing the additional judges, all six current judges must unanimously agree to remove Yoon from office. Any dissenting vote could restore him to power.

Han has stated that he would only approve the appointment of the judges if the ruling People Power Party and the opposition reach a consensus on the nominations.

"The fundamental principle embedded in our Constitution and laws is to refrain from exercising critical presidential powers, such as the appointment of constitutional officials, without consensus between the ruling and opposition parties in the National Assembly, which represents the people," argued the 75-year-old career bureaucrat Han.

If the opposition succeeds in passing the impeachment motion in Friday’s vote, it would mark the first time in democratic South Korea’s history that an acting president has been impeached.

In that case, Finance Minister Choi Sang-mok would assume the duties of acting president.