Prime Minister Shigeru Ishiba of Japan is set to resign, according to the nation’s public broadcaster, in a decision that would plunge the country into deep political uncertainty in a turbulent time domestically and geopolitically.

Japan is grappling with rising right-wing populism at home and heightened unpredictability from its key ally, the United States. Mr. Ishiba, who was elected to lead the governing Liberal Democratic Party just last September, is choosing to step down to prevent a split within the party, the public broadcaster, NHK, reported on Sunday.

Several prominent members of the Liberal Democrats have called for a vote to be held on Monday on whether to hold an extraordinary leadership election, nearly two months after suffering a huge setback in parliamentary elections in which new right-wing populist groups gained support. The blow left the Liberal Democrats, who have led Japan for all but five of the last 70 years, a minority party in both chambers of the Diet, the country’s Parliament.

Mr. Ishiba’s government has also had to manage increasingly tense relations with the United States since President Trump’s re-election. Japan and the United States took months to negotiate a trade deal that left Japanese exports with a 15 percent across-the-board tariff. Even after the agreement was reached, key details remain unresolved, including a Japanese pledge to invest $550 billion in the U.S. economy.

The government said Mr. Ishiba would hold a news conference later Sunday evening.