The main opposition parties have decided to field single candidates in most one-seat districts in this summer’s Upper House election to more effectively face off with ruling party candidates, their members said Wednesday.

Under the agreement involving the Constitutional Democratic Party of Japan, the Democratic Party for the People and the Japanese Communist Party, the parties have agreed to cooperate in 30 out of the 32 constituencies where one seat will be contested. In 14 of the 30 districts, the parties have agreed not to run against independent candidates.

“We are taking an important step toward toppling the government (of Prime Minister Shinzo Abe),” Yukio Edano, leader of the CDP, told reporters after a meeting of the opposition forces.

An election needs to be held as the six-year term for half the members of the House of Councilors will expire on July 28. A total of 124 seats will be up for grabs in the upcoming Upper House race, with voters casting two ballots — one to choose electoral district representatives and one under proportional representation.

The opposition parties are trying to align with each other to avoid the risk of splitting the vote, as media polls continue to show that their support lags behind Abe’s ruling Liberal Democratic Party.

They also agreed during the meeting to coordinate candidates for a possible House of Representatives snap election, which they think could be called by Abe to coincide with the Upper House race.

Abe has not hinted at such a double election so far, and there are more than two years left until the current term of Lower House members expires. But opposition parties may face an uphill battle if a double election takes place, pundits say.

On Wednesday, a close aide of Abe said it is solely up to the prime minister to decide whether to dissolve the Lower House for a snap election, while playing down speculation of a simultaneous vote.

“I work closely with the prime minister but the issue has never been raised,” Deputy Chief Cabinet Secretary Yasutoshi Nishimura said on a radio program. “It doesn’t feel like (a double election is coming) at all.”

LDP election chief Akira Amari also said during a TV program on Monday that he puts the probability of a double election not happening at “99 percent.”

Taking into account the schedule of the ongoing regular Diet session, which ends on June 26, and the time needed for preparation, some ruling party lawmakers say that Abe may have to decide by mid-June whether to go for a double election.

July 21 has so far been seen as the most likely date for the Upper House election, if the current Diet session is not extended.

LDP Diet affairs chief Hiroshi Moriyama on Wednesday said there was no need to consider an extension, telling reporters that deliberations over bills in the Lower House are nearing an end.

Of the 30 districts where the opposition will cooperate in the Upper House election, the CDP will field candidates in seven prefecture-based constituencies, such as Aomori and Miyagi, followed by the DPP’s five.

The JCP will submit candidates in three districts, including two electoral constituencies that are each made up of two neighboring prefectures — Tottori and Shimane, as well as Tokushima and Kochi.

Independent candidates will run in 14 districts. The candidate to run in the remaining district is undecided.