This means that Ishiba no longer has easy control over the upper or lower houses of parliament; last October, the LDP lost control of the more powerful lower chamber, marking a major upset for a party that has ruled Japan for most of its post-World War II history.

Ishiba's ruling Liberal Democratic Party and its junior coalition partner Komeito were short three seats to maintain a majority in the 248-seat upper house in Sunday's vote.

The embattled prime minister said he would remain in office to oversee tariff talks with the United States and other pressing matters.