SEOUL — Washington is walking a fine line between South Korea’s conservative leaders and leftist forces anxious to see President Yoon ousted by a high court accepting the impeachment motion voted by the national assembly.

At the center of American diplomacy here, the acting ambassador, Joseph Yun, has been interacting with the opposition Democratic Party, or Minju, that rammed Mr. Yoon’s impeachment through the assembly after he issued a martial law decree in December that the legislators quickly voted down. With the constitutional court gearing to accept or reject the motion, the ambassador has given the impression of siding with the Minju in its quest for a peace agreement in place of the armistice that ended the Korean War in 1953.