[9] Legislative productivity under the DPJ was particularly low, falling to levels unprecedented in recent Japanese history according to some measures.

[10] However, the DPJ implemented a number of progressive measures during its time in office such as the provision of free public schooling through high school, increases in child-rearing subsidies,[11] expanded unemployment insurance coverage,[12] extended duration of a housing allowance,[13] and stricter regulations safeguarding part-time and temporary workers.

In the 2004 House of Councilors elections, the DPJ won a seat more than the ruling Liberal Democrats, but the LDP still maintained its firm majority in total votes.

Okada resigned the party leadership, fulfilling his campaign promise to do so if the DPJ did not obtain a majority in the Diet.

Although he initially led the party's criticism of the Koizumi administration, particularly in regards to connections between LDP lawmakers and scandal-ridden Livedoor, the revelation that a fake email was used to try and establish this link greatly damaged his credibility.

The DPJ's strong majority in the House of Representatives assured that Hatoyama would be the next prime minister to replace Tarō Asō, leader of the LDP.

Hatoyama was nominated on September 16 and formally appointed later that day by Emperor Akihito in the Tokyo Imperial Palace and formed his Cabinet.

He cited breaking a campaign promise to close an American military base on the island of Okinawa Prefecture as the main reason for the move.

He also mentioned money scandals involving a top party leader, Ozawa, who resigned as well, in his decision to step down.

[24] Hatoyama had been pressured to leave by members of his party after doing poorly in polls in anticipation of the July upper house election.

Prior to the election Kan raised the issue of an increase to Japan's 5 per cent consumption tax in order to address the country's rising debt.

Although Ozawa initially had a slight edge among DPJ members of parliament, local rank-and-file party members and activists overwhelmingly supported Kan, and according to opinion polls the wider Japanese public preferred Kan to Ozawa by as much as a 4–1 ratio.

[35] In September 2010, the government intervened to weaken the surging yen by buying U.S. dollars, a move which temporarily relieved Japan's exporters.

[37] On 24 February 2016, the DPJ announced an agreement to merge with the smaller Japan Innovation Party (JIP) and Vision of Reform ahead of the Upper House elections in the summer,[38][39] with a merger at a special convention agreed for 27 March.

The DPJ wanted to "overthrow the ancient régime locked in old thinking and vested interests, solve the problems at hand, and create a new, flexible, affluent society which values people's individuality and vitality.

"[50] We stand for those who have been excluded by the structure of vested interests, those who work hard and pay taxes, and for people who strive for independence despite difficult circumstances.

[50] The DPJ's policy platforms included the restructuring of civil service, monthly allowance to a family with children (¥26,000 per child), cut in gas tax, income support for farmers, free tuition for public high schools, banning of temporary work in manufacturing,[51] raising the minimum-wage to ¥1,000 and halting of increase in sales tax for the next four years.