As parliament wraps up with debates for the 2025 fiscal year budget, attention in Japan's political world is turning to the summer Upper House election and beyond.

That is prompting questions about whether at least one opposition party could join the Liberal Democratic Party and Komeito ruling coalition after the votes have been counted and, if so, whether there would be a new prime minister and who that person might be.

The current situation, in which the minority government of the LDP-Komeito coalition needs opposition votes to pass legislation, has created concerns about the administration’s long-term stability in dealing with domestic and diplomatic issues. Of particular concern is its relationship with the United States and U.S. President Donald Trump.

While all quarters have been quick to point to the tough political and practical difficulties of bringing in any opposition party as a third coalition partner, that hasn’t stopped questions of how to better achieve a stable parliamentary majority or speculation on what a larger ruling coalition might be the solution.

One scenario is a coalition with all three main opposition parties — the Constitutional Democratic Party of Japan (CDP), Nippon Ishin no Kai and the Democratic Party for the People (DPP) — with Prime Minister Ishiba staying on as its leader.

Scenario 2 is a three-party coalition that includes the CDP, with its head, Yoshihiko Noda, becoming the new prime minister. Noda is a former prime minister.

Scenario 3 would also be a three-party coalition, but involving the DPP and with its chief, Yuichiro Tamaki, taking on the prime minister role.

For the moment, party leaders themselves, with an eye toward doing well in the Upper House election, are unwilling to entertain speculation on what would happen afterward.

Noda said it is more important to concentrate on current political realities than talk about what might happen down the road.

“There is all sorts of speculation going around now, but I don't have anything in mind. I don't think we should be talking about forming a coalition government. We need to decide on the budget for the next Diet session and also settle the issue of corporate donations before the end of the fiscal year,” Noda said on an NTV program last Tuesday, in response to requests for his thoughts on possible new coalition arrangements and prime minister candidates among the opposition such as himself.

Tamaki has repeatedly denied that the DPP is considering joining the LDP in a formal coalition.

Nippon Ishin co-leader Seiji Maehara has warned that being a coalition partner with the LDP-Komeito coalition might look good in the short term, but it could prove fatal to a smaller party in the end.

“If we were to tie up with the LDP and Komeito, we'd be tossed aside if their support rate rises and that would be the end of the road for us. I don't know what the DPP thinks. But from our perspective at Nippon Ishin, we’re extremely negative about this kind of 'supplementary' relationship," Maehara said on the same NTV program.

Political commentator Takuya Nishimura does not see any of the three outlined scenarios as being realistic, given the policy differences of the parties and the different voters that each attracts. Rather, he said, it’s more likely there will be no formal change to the LDP-Komeito coalition after the Upper House election.

For Ishiba, the passage of the budget bill in the Lower House represents a crucial yardstick.

If an outright LDP-Komeito majority isn’t needed in the Lower House, the administration doesn’t need it in the (less powerful) Upper House either, Nishimura said.

“So, Ishiba has no reason to give up the prime minister’s chair,” he added.

Yet a disastrous Upper House election for the LDP would increase calls within the party for a new prime minister, especially among members who belonged to a faction previously led by former Prime Minister Shinzo Abe who strongly dislike Ishiba.

But Nippon Ishin, whose former leaders were close to former LDP prime ministers Abe and Yoshihide Suga, is saying it won’t join a coalition.

The idea of naming Noda or Tamaki as prime minister in any of the scenarios is also fraught with political problems.

“A condition of an LDP-Komeito-DPP coalition is not only agreement on policies but also eliminating electoral district rivalries, which will be very difficult to do. The CDP joining the coalition would likely only happen if there was a natural disaster or war with China, Russia or North Korea,” Nishimura said.

Despite the continued official denials, however, speculation is likely to continue on what kind of expanded coalition government might work best, with the final answer dependent on who wins big, and who loses big, in the Upper House elections.