The main opposition Democratic Party decided Thursday to join forces with the newly established party led by Tokyo Gov. Yuriko Koike in the upcoming lower house election, effectively disbanding itself to form a united front against the ruling Liberal Democratic Party.

The Democratic Party, which has been struggling in opinion polls, will now ask Koike's "Kibo no To" (party of hope) to back its candidates in the Oct. 22 election, said the party's leader Seiji Maehara who assumed his post just three weeks ago.

"I hope you all understand the decision to choose substance over appearance," Maehara told a meeting of party lawmakers held after the House of Representatives was dissolved.

"We want to make Kibo no To bigger and achieve a change of power in this lower house election," said Maehara in a press conference later Thursday.

The Democratic Party will not list its members in the proportional representation blocks of the general election. Of 465 lower house members, 176 will be elected through proportional representation.

The largest opposition party was launched in March 2016 through the merger of the Democratic Party of Japan and a smaller party. The DPJ was in power between 2009 through 2012.

The plan proposed by Maehara signals a major realignment of opposition forces, with his party's group currently holding over 80 seats and Koike's party aiming to field over 100 candidates.

On Wednesday, Koike announced the launch of a "tolerant, reform-minded conservative party" with 14 founding members, most of whom have left the Democratic Party, including Goshi Hosono, a lower house member who served as environment minister.

Speaking at a press conference Thursday originally scheduled to discuss her running of the metropolitan government, Koike said she is still "scrutinizing" how many candidates the party will field, including current Democratic Party members keen to run with her party's backing.

She brushed off speculation that she will quit as governor and run for a parliamentary seat, also stopping short of giving detailed party campaign pledges.

Kazuo Shii, leader of the Japanese Communist Party, which is fielding candidates in most of the 289 single-seat constituencies, said his party will field candidates against Democratic Party members running under the Koike party banner.

Prime Minister Shinzo Abe criticized the Democratic Party's plan, telling a LDP meeting that the rise of such new parties in the past had brought about "turmoil and an economic slowdown."