Opposition lawmakers in South Korea were planning to vote on Friday to impeach the prime minister and acting president, Han Duck-soo, the latest turn in a political crisis that has created a power vacuum in the country.

Mr. Han had been made acting president just earlier this month, after the National Assembly impeached and suspended President Yoon Suk Yeol on Dec. 14 for putting the country under military rule for the first time in 45 years.

Now, barely two weeks into Mr. Han’s tenure as acting president, the main opposition party has filed a motion for his impeachment as well. The move came after Mr. Han refused on Thursday to appoint three judges to fill vacancies in the Constitutional Court, the body that will be deciding whether to reinstate or remove Mr. Yoon.

The opposition has pushed for Mr. Han to sign off on nominees to fill the bench in the nation’s highest court, but Mr. Yoon’s governing party has argued that only an elected president has the power to appoint justices.

At the heart of the matter is how the court might rule on Mr. Yoon’s impeachment. Six or more justices out of the nine-member court must vote in favor of impeachment to remove Mr. Yoon from office. The top court currently has only six justices, after three others retired earlier this year, meaning that the impeachment could be overturned with just one dissenting voice in Mr. Yoon’s trial, which is scheduled to start on Friday.

Mr. Han said in a televised address that he would hold off on appointing the nominees until the rival parties — that is, Mr. Yoon’s People Power Party and the opposition bloc comprising the Democratic Party and other smaller parties — came to an agreement on whether he had the authority to do so as the acting president.