While in university, he began to assist Kazunori Yamanoi, and, in 1996, when the Democratic Party formed, served as permanent secretary inside the Kyoto Prefecture.

During the Democratic Party of Japan government's from 2009-2012, he served as Parliamentary Vice-Minister of the Cabinet Office and was in charge of the Administrative Reform Council.

When the Democratic Party split in 2017, he joined Kibō no Tō, where he served as the chairman of the National Assembly Committee from 2017 to 2018.

[3] His father, Norio (Who died in 2011 at the age of 74), served as the neighborhood chairman,[4] and later as a member of the Ishikari City Council.

[9] During the Kyoto mayoral election held in the same year, he organized a lecture event inviting candidates to speak.

[10] After his defeat, he returned to his previous political activities while working part-time as a daycare worker and elderly caretaker.

[16] In February 2008, during his second term, he worked with Taro Kono, Masahiko Shibayama, Kenichi Mizuno, and Koichi Yamauchi of the LDP along with Sumio Mabuchi and Goshi Hosono of the DPJ to introduce new rules regarding the operation of the Diet, including a year-round Diet sesson.

[17] In May of the same year, he also proposed a law to give compensation to former BC-class war criminals from Taiwan and the Korean Peninsula.

[10] Izumi defeated Koichiro Shimizu again in the 2009 election, in which the Democratic Party of Japan won a majority and ascended Yukio Hatoyama to become prime minister.

[20] He also became the leader of the Food SOS Response Project, which was launched by the Consumer Affairs Agency to address health issues such as Econa, an edible coconut oil which had concerns over its safety for human consumption.

[9] In the 2011 Democratic Party of Japan leadership election, which was won by Yoshihiko Noda, Izumi was again one of Seiji Maehara's endorsers.

[23] In the 2012 election, which ended the DPJ's brief stay in power by a landslide, Izumi lost to Kensuke Miyazaki in the third ward.

[27] In the 2015 Democratic Party leadership election, Izumi endorsed Goshi Hosono, who narrowly lost to Katsuya Okada.

Since the LDP did not run a candidate, he won by a 40 point margin, capturing a safe majority of 65.42% against his closest opponent, a member of Nippon Ishin no Kai.

[30] In the 2017 Democratic Party leadership election, he backed Maehara and served as secretary-general for his campaign office.

Izumi ran for party leader again following this, competing with three others; Seiji Osaka, Junya Ogawa, and Chinami Nishimura.

[40] Izumi came first in the first round with 33.0% of member votes, but was forced into a runoff with Seiji Osaka, who was closer to Edano during his time as party leader.

[39] Izumi has made it clear that he will be less confrontational with the government and will change the party's image to one that is more "proposal-orientated" than opposition focused.

[45] In the 2022 House of Councilors election, where the CDP took a policy focused approach, the party lost six seats, and in terms of proportional representation, received less votes than Nippon Ishin no Kai.

[46] Izumi also revamped his executive council, bringing on Katsuya Okada as secretary-general, Akira Nagatsuma as chairman of the Policy Research Council, and Jun Azumi as chairman of the National Relations Committee, all experienced politicians who assisted the Democratic Party of Japan and served in the 2009-2012 governments.

[46] In addition, the party switched from its prior policy focused approach to returning to confrontation with the government, and began to cooperate in the Diet with Nippon Ishin no Kai.

Another one conducted during the 2005 election gave a response by him that this ideal should continue to be shared by the Japanese people, but also that it was important to revise the constitution in line with the new era.

[54] In another survey conducted before the 2012 election, he responded and stated the need for environmental and privacy rights, the supremacy of the House of Representatives, and the need to enshrine the Japan Self-Defense Forces in Article 9 of the Japanese Constitution.

[59] In a survey in 2021, he advocated participating as an observer in the Treaty on the Prohibition of Nuclear Weapons, and called for Japan to "pave the way" on the issue.

[62] In January 2022, he joined a "Self-Defenses Forces Supporters League" inside the Constitutional Democratic Party.