Nagatsuma gained nationwide prominence after exposing scandals involving Japan's bureaucracy, including widespread mishandling of the national pension records and the misuse of public funds for kickbacks. He earned the nickname Mr Pension for his investigations into these scandals.[2]
His experience and expertise on pensions led him to be appointed to the Minister of Health, Labour and Welfare during the administrations of Prime Ministers Yukio Hatoyama and Naoto Kan. After Prime Minister Kan's first cabinet reshuffle, he moved to the DPJ party leadership and became the first vice secretary-general of the party. He continued to be active in the Diet after leaving his cabinet post and became the chairman of the Welfare and Labour Committee in the House of Representatives in 2012.
In December the same year, a general election was held and the LDP returned into power. Amidst large swings against the DPJ, Nagatsuma managed to hold his seat. Nagatsuma and Akihisa Nagashima were the only DPJ representatives in Tokyo to win single-seat constituencies in the election. Nagashima was defeated in his district in 2014, leaving Nagatsuma as the only DPJ member with a constituency seat in Tokyo.
Nagatsuma continued to rise through the ranks of the party after the 2012 defeat. He became DPJ's deputy secretary-general in May 2013.[3] After then-party leader Banri Kaieda was defeated in the 2014 election, Nagatsuma entered the race to succeed him, but lost to Katsuya Okada. Okada then appointed Nagatsuma to be the party's deputy leader, a position he held even after the merger with Japan Innovation Party.[4]
After the election of Seiji Maehara as a president of the post-merger Democratic Party (DP) in September 2017, Nagatsuma was appointed as the party's election campaign committee and its Tokyo chapter chairman.[5][6]
Less than three weeks after the conclusion of the leadership race, Prime Minister Shinzo Abe called for a snap general election in October. Earlier on the day of the PM's declaration, Tokyo governor Yuriko Koike formed Kibō no Tō, intended to be a conservative alternative to the LDP. Many conservative-leaning DP members migrated en masse to Kibō after Maehara allowed DP members to run in the election under Kibō's banner. However, Koike imposed an ideological test that effectively barred liberal DP members from joining Kibō. This spurred DP deputy president Yukio Edano to form the CDP just three weeks before the election. Nagatsuma, a founding member, was tapped as the party's deputy leader.[7]
The burgeoning CDP campaign received a surge of support in the run-up to election day. CDP made significant gains in the election across the country, becoming the country's largest opposition party. The surge was also replicated in Tokyo, where the campaign was led by Nagatsuma. LDP's majorities were cut in seats where CDP were contesting. Some were overturned, as in the case of former DPJ leaders Kan and Kaieda, who contested under the CDP banner and regained their seats.[8] After the election, Nagatsuma became the CDP's Tokyo chapter head.[1]
Following the 2025 Tokyo prefectural election, 5 independents joined the CDP caucus, making the party the second-largest party in the Assembly, only behind Tomin First and ahead of the LDP.[9]
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