South Korea’s opposition leader offered Sunday to work with the government to ease the political tumult as officials sought to reassure allies and markets, a day after the opposition-controlled parliament voted to impeach conservative President Yoon Suk Yeol over a short-lived attempt to impose martial law.

Liberal Democratic Party leader Lee Jae-myung, whose party holds a majority in the National Assembly, urged the Constitutional Court to rule swiftly on Yoon’s impeachment and proposed a special council for policy cooperation between the government and parliament.

Yoon’s powers have been suspended until the court decides whether to remove him from office or reinstate him. If Yoon is dismissed, a national election to choose his successor must be held within 60 days.

Lee, who has led a fierce political offensive against Yoon’s embattled government, is seen as the frontrunner to replace him. He lost the 2022 presidential election to Yoon by a razor-thin margin.

He told a televised news conference that a swift court ruling would be the only way to “minimize national confusion and the suffering of people.”

The court will meet to discuss the case Monday, and has up to 180 days to rule. But observers say that a court ruling could come faster. In the case of parliamentary impeachments of past presidents — Roh Moo-hyun in 2004 and Park Geun-hye in 2016 — the court spent 63 days and 91 days respectively before determining to reinstate Roh and dismiss Park.

Lee also proposed a national council where the government and the National Assembly would work together to stabilize state affairs, and said his party won’t seek to impeach Prime Minister Han Duck-soo, a Yoon appointee who’s now serving as acting president.

“The Democratic Party will actively cooperate with all parties to stabilize state affairs and restore international trust,” Lee said. “The National Assembly and government will work together to quickly resolve the crisis that has swept across the Republic of Korea.”

It’s unclear if Lee’s proposed council will be realized.

In a meeting with the parliament speaker, who touched upon Lee’s idea, Han said he will closely cooperate and communicate with the National Assembly but didn’t specifically say whether the government intends to join the council. Kweon Seong-dong, floor leader of the ruling People Power Party, separately criticized Lee’s proposal, saying that it’s “not right” for the opposition party acting like the ruling party.

Kweon, a Yoon loyalist, said that his party will use existing PPP-government dialogue channels “to continue to assume responsibility as the governing party until the end of President Yoon’s term.”

The Democratic Party has already used its parliamentary majority to impeach the justice minister and the chief of the national police over the martial law decree, and had previously said it was also considering impeaching the prime minister.

Upon assuming his role as acting leader, Han ordered the military to bolster its security posture to prevent North Korea from launching provocations. He also asked the foreign minister to inform other countries that South Korea’s major external policies will remain unchanged, and the finance minister to work to minimize potential negative impacts on the economy from the political turmoil.

On Sunday, Han had a phone call with U.S. President Joe Biden, discussing the political situation in South Korea and regional security challenges including North Korea’s nuclear program. Biden expressed his appreciation for the resiliency of democracy in South Korea and reaffirmed “the ironclad commitment” of the United States, according to both governments.

Yoon’s Dec. 3 imposition of martial law, the first of its kind in more than four decades, lasted only six hours, but has caused massive political tumult, halted diplomatic activities and rattled financial markets. Yoon was forced to lift his decree after parliament unanimously voted to overturn it.

Yoon sent hundreds of troops and police officers to the parliament in an effort to stop the vote, but they withdrew after the parliament rejected Yoon’s decree. No major violence occurred.

Opposition parties have accused Yoon of rebellion, saying a president in South Korea is allowed to declare martial law only during wartime or similar emergencies and would have no right to suspend parliament’s operations even in those cases.

Yoon has rejected the charges and vowed to “fight to the end.” He said the deployment of troops to parliament was aimed at issuing a warning to the Democratic Party, which he called an “anti-state force” that abused its control of parliament by holding up the government’s budget bill for next year and repeatedly pushing to impeach top officials.

Law enforcement institutions are investigating Yoon and others involved in the martial law case over possible rebellion and other allegations. They’ve arrested Yoon’s defense minister and police chief and two other high-level figures.

Yoon has immunity from most criminal prosecution as president, but that doesn’t extend to allegations of rebellion or treason. He’s been banned from leaving South Korea, but observers doubt that authorities will detain him because of the potential for clashes with his presidential security service.