Japan’s main opposition parties have decided to field single candidates in most of the one-seat districts in the upcoming upper house election in the summer to 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 join hands in 30 out of the 32 constituencies where one seat will be contested.

“We are taking an important step toward toppling the government (of Prime Minister Shinzo Abe),” Yukio Edano, leader of the Constitutional Democratic Party of Japan, 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 Councillors 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 voters 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 House of Representatives election, which they fear could be called by Abe to coincide with the upper house race.

Abe has not hinted at such a double election so far, with more than two years left until the current term of the 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 election.

“I work closely with the prime minister but the issue has never been raised,” Deputy Chief Cabinet Secretary Yasutoshi Nishimura said in 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 probable date for the upper house election to be held, if the current parliamentary session is not extended.

LDP Diet affairs chief Hiroshi Moriyama on Wednesday denied the need to consider an extension, telling reporters that deliberations of bills at the lower house are nearing an end.

For the 30 districts in the upper house election, the Constitutional Democratic Party of Japan will field candidates in seven prefecture-based constituencies, such as Aomori and Miyagi, followed by the Democratic Party for the People’s five.

The Japanese Communist Party will file candidates in three districts, including two electoral constituencies which are each made up of two neighboring prefectures — Tottori and Shimane prefectures as well as Tokushima and Kochi prefectures.

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

The Japan Innovation Party is not part of the agreement to field single candidates.