Japanese Prime Minister Shigeru Ishiba's ruling coalition has failed to secure a majority in Sunday's election for the 248-seat upper house of parliament, Japan's public NHK TV outlet said.

Ishiba's Liberal Democratic Party (LDP), already in a minority in the lower house since last October's snap election, faced mounting public discontent over inflation, political scandals and a surge in anti-immigration sentiment.

Why Ishiba won't resign as prime minister

The LDP and its junior partner Komeito needed to win 50 of the 125 contested seats in addition to the 75 they already have. They fell short, winning only 46 of the seats. 

In recent years, such an electoral defeat has usually led to a prime minister's resignation. Ishiba, however, vowed to stay on to lead Japan through the difficult terrain of striking a trade deal with the US in the face of looming tariffs.

"We are engaged in extremely critical tariff negotiations with the United States ... we must never ruin these negotiations," he told a press conference after vowing to stay on as party leader through the trade talks.

The loss is another blow to Ishiba's coalition, which is now a minority in both houses following its October defeat in the lower house election. It is the first time the LDP has lost a majority in both houses of parliament since the party's founding in 1955.

The poor performance in the election will not immediately trigger a change of government because the upper house lacks the power to file a no-confidence motion against a leader. Despite his determination to stay on, Ishiba could face calls from within his party to step down or find another coalition partner.

Rise of the 'Japanese First' party

The surge of the right-wing populist Sanseito party complicated the election.

Once a fringe movement, spreading conspiracy theories about vaccinations and global elites during the COVID-19 pandemic on YouTube, the party is now gaining momentum with its "Japanese First" rhetoric and criticism of immigration, globalism, and foreign capital.

Sanseito is expected to win 14 seats in the upper house, appealing especially to young male voters.

Its leader, Sohei Kamiya, has drawn comparisons to Trump and Germany's AfD for his anti-establishment stance and social media reach.

According to AFP news agency, a Sunday interview by Russia's Sputnik news agency with a Sanseito candidate courted controversy and prompted Kamiya to deny ties to Russia.

The European Union has banned Sputnik and other Russian state media for "disinformation and information manipulation."

Edited by: Alex Berry, Wesley Dockery