SEOUL, South Korea — For her nearly four years in office, President Park Geun-hye of South Korea cooperated closely with the United States, particularly when it came to dealing with her volatile neighbor, North Korea.

A vote on Friday to impeach her now throws both her country and American policy in the region into deep uncertainty, as the North’s nuclear program advances and the incoming administration of Donald J. Trump deliberates over whether to adjust Washington’s stance on how to best contain North Korean aggression.

Ms. Park, a conservative, had adopted a tough approach toward the North, focusing on stronger sanctions. Her administration had also agreed to deploy an American advanced missile defense system that infuriated the Chinese.

Yet her deep unpopularity — the result of a scandal over influence-peddling that led members of her own party to want to oust her — increases the odds that the next election will be won by an advocate of friendlier relations with China.