SEOUL — South Korea’s impeached president, Yoon Suk-yeol, is saying he is “sorry” about the turmoil he created by declaring martial law on December 3 but blames his leftist foes for his country’s worst political crisis in decades.

The conservative Mr. Yoon, fighting not just for his political survival but to avoid imprisonment for his ill-conceived martial law declaration, pleaded his case at the end of hearings by the constitutional court that’s expected to rule in two weeks on his impeachment by the national assembly.

Mr. Yoon justified his declaration of martial law as an attempt to stop the assembly, dominated by the opposition Minju, or Democratic party, from obstructing his legislative agenda ever since his election by a slim margin over the Minju leader in March 2022. 

“I am sorry but thankful to the people,” he said, as quoted by Seoul’s Yonhap News. He quickly added the reason for the apology: “the reality of not being able to do my work in the time given to me by the people.” 

Mr. Yoon also offered what he believed would appeal to the widespread view that the five-year presidential term is both too long and too short. There would, he said, “be no reason to cling to the remainder of my term” if Korea’s “democracy constitution,” enacted after protests in 1987 against dictatorial rule, is amended. The inference was that a president would serve a briefer term but could seek a second term.

“If a constitutional amendment and political reform are pursued correctly, I believe the separated and divided people will unite in the process,” Seoul’s Yonhap News quoted him as saying.

As Mr. Yoon spoke, several thousand right-wing demonstrators, waving American and South Korean flags, shouted and screamed denunciations of both the left-leaning Minju, for undermining his government, and the court, several of whose eight justices are seen as pro-Minju and sure to vote to oust him. His future hangs on the sentiments of the four remaining members of the eight-judge panel, which needs six votes to oust him. The panel is supposed to have nine judges, but one seat is vacant.

Whatever the court decides, the outcome is sure to trigger widespread protests by the losing political party. The latest polls indicate that a slim majority backs Mr. Yoon’s People Power Party over the Minju, while an equally slim majority favors the Minju leader, Lee Jae-myung, the loser in the 2022 election.

“Yoon didn’t have a choice but to declare martial law,” a young protester, Park Jun-yong, said while part of a crowd down the street from the court, protected by rows of police buses and hundreds of policemen. “Everybody will rise up if there is an adverse ruling.”

Mr. Lee, despite facing corruption charges in real estate and other scandals, is fairly certain to run in a “snap election” that would have to be held 60 days after the ouster of the president.

Stripped of the power but not the title of president, Mr. Yoon was jailed on the charge of “insurrection” for having declared martial law, which the Minju voted down three hours after he issued his declaration. His defense and interior ministers as well as about 30 army and police officers were also jailed while charged for their roles in the fracas surrounding the assembly.

The rightist protesters have been gaining strength and numbers since the nights of December 3 and 4, when soldiers surrounded the assembly and helicopters hovered overhead as the nation watched live on TV. No shots were fired, and no one was injured in the mayhem. 

 “I’m not worried about the future,” another protester, Jun Anuk, said, confident that four members of the court would vote for Mr. Yoon, after which the district court would dismiss the charge of insurrection, freeing him to return to work as president. Like many rightists, he blamed China for elections that Mr. Yoon suspected were rigged, enabling the Minju to control the assembly. “Korea and the U.S. still have the same enemies,” he told the Sun. 

In that spirit, the rightists plan their biggest protest so far on Saturday, the anniversary of the bloody uprising of March 1, 1919, in which Korea’s colonial Japanese rulers massacred about 10,000 Koreans. Several hundred thousand demonstrators, as always waving the flags of both America and South Korea, are expected to swarm the main avenue through central Seoul. 

The Minju frames Mr. Yoon’s attempt at imposing martial law in the context of the military dictatorship that prevailed until 1987. Yonhap reported that the leader of the assembly’s impeachment committee, Jung Chung-rae, told the constitutional court that Mr. Yoon had to go “for the sake of democracy and national progress.”