The effort to impeach Prime Minister Han Duck-soo, who has been acting president since Yoon was impeached on December 14, threatens to intensify the political crisis gripping Asia's fourth-largest economy and one of its most vibrant democracies.
The unexpected martial law decree and swift political fallout shocked the nation and economic markets, unsettling key allies the United States and Europe which had seen Yoon as a staunch partner in global efforts to counter China, Russia, and North Korea.
The plan for a vote to impeach Han was unveiled on Thursday by the main opposition Democratic Party after he declined to immediately appoint three justices to fill vacancies at the Constitutional Court, saying it would exceed his acting role.
Han Duck-soo could also be impeached. (AP PHOTO)
After Yoon's impeachment, the DP had said in the interest of national stability it would not pursue impeaching Han over his role in the martial law bid.
But the party has since clashed with the Yoon-appointed prime minister over the justices, as well as bills calling for special prosecutors to investigate the president.
On Thursday Han said it was beyond his remit as a caretaker president to appoint the justices without bipartisan agreement.
A party spokesman said Han's refusal amounted to an abuse of power aimed at obstructing Yoon's trial, adding that the prime minister was himself "a key suspect in the rebellion".
The leader of Yoon's People Power Party, Kwon Young-se, told reporters that if Han was impeached, that could trigger a new financial crisis, the Yonhap news agency said.
The shock martial law decree and swift political fallout shocked the nation and economic markets. (AP PHOTO)
Yoon cited a high number of impeachment votes and other obstructionist moves by the DP as part of his justification for trying to impose martial law. He also later said it was needed to investigate questions over election security.
The vote to determine Han's fate comes as the Constitutional Court is set on Friday to hold its first hearing in a case that will decide whether Yoon is reinstated or permanently removed from office.
The court has 180 days to decide whether to reinstate Yoon or remove him. In the latter scenario, a new presidential election would be held within 60 days.
Yoon is not required to attend the hearing and it is unclear if anyone from his legal team will be there.
In contrast to South Korea's two previous impeached presidents, Yoon has refused to receive or acknowledge court communications so far.
On Thursday a court spokesperson said the hearing would be held regardless of his team's participation, but she did not comment on whether the president would eventually be compelled to respond.
If Han is impeached, the finance minister will assume the acting presidency.
The Democratic Party has majority control of parliament, but there is disagreement between the parties and some constitutional scholars over whether a simple majority or a two-thirds vote is needed to impeach the acting president.
On Thursday the South Korean won weakened to its lowest since March 2009 in holiday-thinned trading amid the US dollar's continued rally.
Yoon shocked his country and the world with a late-night announcement on December 3 that he was imposing martial law to overcome political deadlock and root out "anti-state forces".
The military deployed special forces to the national assembly, the election commission, and the office of a liberal YouTube commentator.
It also issued orders banning activity by parliament and political parties, as well as calling for government control of the military.
But within hours 190 lawmakers had defied the cordons of troops and police and voted against Yoon's order. About six hours after his initial decree, the president rescinded the order.
