The outlook for Prime Minister Shigeru Ishiba's government is likely to hinge on a small opposition party that in an electoral breakthrough secured 28 seats in the 465-member House of Representatives.

The Democratic Party for the People quadrupled its seats in the October general election by garnering support from younger voters with what some political analysts called populist pledges, such as measures to boost disposable income as inflation weighs on domestic demand.

As Ishiba's Liberal Democratic Party and its junior coalition partner struggle to fulfill their promises after losing their majority in the lower house, the DPP has engaged in policy-by-policy negotiations with the ruling bloc, further increasing its clout.

Expectations are growing that the party's popularity may be peaking amid increasing uncertainty over its ability to push through its priorities as its leader, Yuichiro Tamaki, temporarily steps back from public engagements through March following revelations of an affair shortly after the election.

Rintaro Nishimura, an associate at the Asia Group Japan business advisory firm, said Tamaki and the DPP "haven't really solidified" their new base, adding their supporters "could shift to another party any time they want."

The DPP, a successor to the now-defunct left-leaning Democratic Party of Japan, which governed the country for about three years until December 2012, shifted its focus from its core base of labor union supporters with a social-media-driven campaign.

The simplicity of the headline policy, coupled with the effective use of Tamaki's YouTube channel, drew in younger voters -- a typically overlooked demographic in aging Japan, where older generations wield greater influence in national elections.

After a stump speech by Tamaki, a former Finance Ministry bureaucrat, during the election campaign, multiple first-time voters said they felt drawn to the party. Tatsuhiro Fukuda, a 19-year-old university student, said the DPP "is closest to my way of thinking."

"I first heard him speak online and thought he was someone I could trust," he said, adding Tamaki's key policy of raising the income tax threshold from the current 1.03 million yen to drive up take-home pay would help his parents.

A Kyodo News survey showed the DPP received support from people in their 20s, up 11 points to 15 percent, and from those in their 30s, up 9 points to 12 percent, compared to the 2021 general election that allowed the LDP to control the lower house.

According to another poll in mid-December, the support rate for the main opposition party, the Constitutional Democratic Party of Japan, stood at 11.3 percent, overtaken by the DPP at 12.6 percent.

LDP policy chief Itsunori Onodera said the ruling coalition has responded to the requests of the DPP "as much as possible," acknowledging that it would be almost impossible for Ishiba to move ahead with his policy without the backing of Tamaki's party.

However, the DPP has also faced difficulties in promoting its policies, including pushing up the income tax threshold, given that Ishiba, who served as Japan's first minister for regional revitalization, is committed to shoring up flagging local economies.

Official estimates show that fully implementing the DPP's proposal would cost the national and municipal governments a total of 7.6 trillion yen in lost revenues annually. Prefectural governors have urged Ishiba to safeguard local revenues.

Koji Nakakita, a professor of politics at Chuo University in Tokyo, said, "I have doubts about how long this popularity can last, and the trigger for that will be the negotiations over the income tax policy," referring to the DPP.

Nakakita said the DPP is too unprepared to achieve its goals. The party based its ambitious proposal to raise the tax threshold on how much the minimum wage has increased since 1995, while the LDP and its coalition partner, Komeito, are arguing for more modest adjustments.

"They had no reason to choose such a large increase. Although small parties make irresponsible promises to survive, this one unexpectedly gathered support," he said, adding the DPP should suggest a smaller revision by tracking the consumer price index.

Takahide Kiuchi, executive economist at Nomura Research Institute, echoed the view, saying that the DPP "should not simply demand a large tax cut, but instead take a broader perspective and remain flexible in its discussions with the ruling parties."

Amid signs of an impasse, the LDP has moved to curb the DPP's leverage in advancing legislation by inviting the 38-seat Japan Innovation Party to join discussions on expanding free education.

Tamaki is also suspending his role as chief for the time being after the party's ethics committee ruled that he damaged its image, even though his efforts have significantly contributed to boosting support for the DPP since its founding in 2018.

"It's hard to find anyone else in the party who can command the same level of popularity," Nakakita said.