In the 2007 Japanese House of Councillors election, he ran as a Democratic Party candidate from Shiga at-large district and was elected after defeating Hidetoshi Yamashita [ja], an incumbent Liberal Democratic Party member.[1]
On June 9, 2010, he was appointed Parliamentary Vice-Minister for Foreign Affairs by Kan Cabinet and worked under Foreign Minister Seiji Maehara and his successor, Foreign Minister Takeaki Matsumoto.[2]
On December 15, 2010, he attended the United Nations Security Council Ministerial Meeting on Iraq in New York as a representative of the Japanese government. Three United Nations Security Council resolution were adopted at the meeting, and Tokunaga has also made a speech on the SDF's contribution to Iraq's reconstruction.[3] He visited Middle Eastern countries from January 10 to 11 of the following year. In Jordan, he met with Prime Minister Samir Rifai and signed an official letter of exchange with Minister of Planning and International Cooperation Hassan to improve water supply in southern Jordan.[4]
In the 2013 Japanese House of Councillors election, he sought re-election, but lost to Liberal Democratic Party candidate Takeshi Ninoyu [ja] (son of Satoshi Ninoyu).[5]
In the 2014 Japanese general election, he ran as a Democratic Party candidate from Shiga 4th district but lost to Takaya Mutō, an incumbent Liberal Democratic Party member.[6]
In 2016, when the Democratic Party merged with the Japan Innovation Party and the Democratic Party was formed, Tokunaga joined this party.
In 2017, the Democratic Progressive Party was divided into the right-wing Kibō no Tō and the left-wing Constitutional Democratic Party. Tokunaga joined the Kibō no Tō.[7]
In the 2017 Japanese general election, he ran as a Kibō no Tō candidate from Shiga 4th district, but lost to Hiroo Kotera of the Liberal Democratic Party, who ran on behalf of Mutō, who retired due to a money scandal over stocks.[8]
The Kibō no Tō merged with the Democratic Party to form the Democratic Party for the People. Tokunaga joined the Democratic Party for the People and became the head of Shiga 4th district branch of the Democratic Party for the People on December 12, 2018.[9]
On September 15, 2020, the former Constitutional Democratic Party and the former Democratic Party for the People joined together to form a new "Constitutional Democratic Party".[10] Tokunaga joined the new party.
In the 2021 Japanese general election, he ran as a Constitutional Democratic Party candidate from Shiga 4th district, but lost again to Hiroo Kodera of the Liberal Democratic Party. However, since the Constitutional Democratic Party won three seats in the Kinki proportional representation block and Tokunaga had the third Sekihairitsu (83.119%), he was elected by Kinki proportional representation block.[11][12] In the 2021 Constitutional Democratic Party of Japan leadership election (held on November 30) following the resignation of Yukio Edano, he endorsed Kenta Izumi.[13]
