TOKYO (AP) -- Prime Minister Shigeru Ishiba on Wednesday vowed to remain in power to oversee the implementation of a new Japan-U.S. tariff agreement, despite media speculation and growing calls for him to resign after a historic defeat of his governing party.

Ishiba met with heavyweights from his Liberal Democratic Party, or LDP, and former Prime Ministers Taro Aso, Fumio Kishida and Yoshihide Suga at party headquarters.

He told reporters afterward that they didn't discuss his resignation or a new party leadership contest, but only the election results, voters' dissatisfaction and the urgent need to avoid party discord.

Despite his business-as-usual demeanor, Ishiba is under increasing pressure to bow out after the LDP and junior coalition partner Komeito lost their majority in Sunday's election in the 248-member upper house, the smaller and less powerful of Japan's two-chamber parliament, shaking his grip on power.

It came after a loss in the more powerful lower house in October, and so his coalition now lacks a majority in both houses of parliament, making it even more difficult for his government to pass policies and worsening Japan's political instability.

Ishiba says he intends to stay on to tackle pressing challenges, including tariff talks with the U.S., so as not to create a political vacuum despite calls from inside and outside his party for a quick resignation.

Ishiba "keeps saying he is staying on. What was the public's verdict in the election all about?" said Yuichiro Tamaki, head of the surging Democratic Party for the People, or DPP.

At the LDP, a group of younger lawmakers led by Yasutaka Nakasone started a petition drive seeking Ishiba's early resignation and renewal of party leadership.

"We all have a sense of crisis and think the election results were ultimatum from the voters," he said.

Japanese media reported that Ishiba is expected to soon announce plans to step down in August.

The conservative Yomiuri newspaper said in an extra edition on Wednesday that Ishiba had decided to announce his resignation by the end of July after receiving a detailed report from his chief trade negotiator, Ryosei Akazawa, on the impact of the U.S. tariffs on the Japanese economy, paving the way for a new party leader.

Ishiba denied the report and said that he wants to focus on the U.S. trade deal, which covers more than 4,000 goods affecting many Japanese producers and industries. He welcomed the new agreement, which places tariffs at 15% on Japanese cars and other goods imported into the U.S. from Japan, down from the initial 25%.

Still, local media are already speculating about possible successors. Among them are ultraconservative former Economic Security Minister Sanae Takaichi, who lost to Ishiba in September. Another conservative ex-minister, Takayuki Kobayashi, and Agriculture Minister Shinjiro Koizumi, the son of former popular Prime Minister Junichiro Koizumi, are also seen as potential challengers.

In Sunday's election, voters frustrated with price increases exceeding the pace of wage hikes, especially younger people who have long felt ignored by the ruling government's focus on senior voters, rapidly turned to the emerging conservative DPP and right-wing populist Sanseito party.

None of the opposition parties have shown interest in forming a full-fledged alliance with the governing coalition, but they have said they are open to cooperating on policy.

People expressed mixed reaction to Ishiba, as his days seem to be numbered.

Kentaro Nakamura, 53, said that he thought it's time for Ishiba to go, because he lacked consistency and did poorly in the election.

"The (election) result was so bad and I thought it would not be appropriate for him to stay on," Nakamura said. "I thought it was just a matter of time."

But Isamu Kawana, a Tokyo resident in his 70s, was more sympathetic and said if it wasn't Ishiba who was elected prime minister last year, the result would have been the same.

"I think he got the short end of the stick," Kawana said.

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Reeno Hashimoto contributed to this report.