TOKYO – Newly minted Japanese Prime Minister Naoto Kan’s embattled Democratic Party suffered a resounding defeat in Japan’s parliamentary elections Sunday, a blow that threatened to further weaken Kan’s already tenuous month-long hold on power.

The Democratic Party of Japan won fewer than 50 seats, well short of the 54 needed for the Democrats and their tiny coalition partner, the People’s New Party, to keep their combined majority in the parliament’s upper house, according to exit polls conducted by Japan’s public broadcaster and all major TV networks. Official results are expected today.

While the Democrats hold a majority in Japan’s more powerful lower chamber, Sunday’s poor poll showing undermines their ability to control the national agenda in the world’s second-biggest economy, beset by massive public debt and a foreign policy malaise typified by the lingering dispute over the continuing presence of U.S. troops on the southern island of Okinawa.

Kan’s party will now need to seek new coalition partners to regain control of the upper house.

The defeat also left Kan — who took office in June after his predecessor, Yukio Hatoyama, quit after only eight months — increasingly vulnerable to a challenge from inside his own party.

Analysts had called Sunday’s vote a litmus test for Kan, a former finance minister who had sought to distance himself from Hatoyama. The latter was labeled as ineffectual after he failed to deliver on a campaign promise to move a major U.S. military base off Okinawa’s main island.

Kan pledged to stick to an agreement with Washington to move U.S. Marine Corps Air Station Futenma to a less crowded part of Okinawa despite public cries to move the troops off the island entirely.

Kan, 63, took office with a 60 percent approval rating. But he quickly stumbled, pushing a plan to double the sales tax to 10 percent, his solution to rein in a public debt that has grown to twice the size of Japan’s nearly $5 trillion economy.