Lee Jae-myung, who an exit poll projects will be the next leader of South Korea, would take office as one of the most powerful presidents the country has chosen in recent decades.

Besides the presidency, where most of the political power is concentrated, he would have the National Assembly on his side as his Democratic Party has a ​large majority of the seats there.

Observers in and outside South Korea wonder: Will he use the immense power to heal his deeply divided country and revive its sputtering economy, as he said he would​? Or will he seek political revenge,​ aggravating political polarization and alienating its ally, the United States, as his enemies said he would?

Here is a glance at what Mr. Lee​, who calls himself a “pragmatist,” has said on key issues:

Foreign Policy

Mr. Lee would represent a break from his impeached predecessor, Yoon Suk Yeol. Mr. Yoon was more confrontational toward North Korea and China, and he improved ties with Japan, a historic enemy, despite outcries in South Korea. He aligned Seoul more firmly with Washington as the strategic competition between the United States and China intensified.

Mr. Lee said he would try to repair his country’s strained ties with China and North Korea while maintaining a strong military alliance with the United States, which he said should remain the bedrock of South Korean diplomacy. He has said Mr. Yoon was too “submissive” toward Japan and too “antagonistic” toward China.

“Cooperation with the United States and Japan is essential,” Mr. Lee said. “But we should not put all our eggs in one basket.”