日本新党
Koike was elected to the House of Councillors in 1992 as a member of the Japan New Party. She was then elected to the House of Representatives in 1993, representing the Hyogo 2nd district. In 1996, she was re-elected to the House of Representatives, this time representing the Hyogo 6th district for the New Frontier Party. She held this seat in the 2000 election as a candidate of the New Conservative Party. She joined the Liberal Democratic Party in 2002. She also has been a regular contributor to Project Syndicate since 2010.