While still a member of the Liberal Democratic Party (LDP), Koike began laying the groundwork for a new political party in late 2016, when she established the Kibo no Juku ("Academy of Hope") program to train potential political candidates.

[8] On 31 May 2017, in advance of the upcoming local elections, she resigned from the LDP, officially becoming the new party's leader and forming an alliance with Komeito in an effort to secure a governing majority in Tokyo's parliament.

[9] On 3 July 2017, the alliance took a majority in the prefectural election, pushing out the Liberal Democratic Party with a combined 79 seats of the 127-seat assembly.

[12] Koike formed a sister national party, Kibō no Tō, in preparation for the 2017 general election.

In the 2021 Tokyo prefectural election, Tomin First lost a total of 15 seats, losing its status as the largest party in the Assembly to the LDP.