For six weeks, South Korea has lurched through its worst political crisis in decades, throwing the resilience of the country’s democracy into question. On Tuesday, it takes the biggest step toward a resolution, when the Constitutional Court begins deliberating whether to remove or reinstate the country’s impeached president.

The eight justices on the court will be the final arbiters on the fate of President Yoon Suk Yeol, who was impeached and suspended from office on Dec. 14 by the National Assembly for his short-lived declaration of martial law 11 days earlier.

The stakes are high. Rival groups of citizens have rallied for weeks, some in front of the court, either calling for Mr. Yoon’s ouster or demanding his return to office. Hard-liners on both sides have warned of “civil war” if the court does not rule in their favor.

If Mr. Yoon is removed, it will be another crushing blow to the country’s conservative camp: He will be the third conservative president in a row to be ousted, imprisoned or both before or after their term ended.

But if the deeply unpopular leader is allowed to return to office, it could set a precedent for future leaders to use martial law as a political tool, said Ha Sang-eung, a professor of political science at Sogang University in Seoul.