A former executive of JP Morgan Securities Japan, he was elected to the House of Councillors in 2010 as a member of Your Party for the Kanagawa at-large district, and he later joined the Liberal Democratic Party House of Councillors parliamentary group.

[1] Nakanishi ran for mayor of Yokohama in 2009 as an independent, but effectively supported by the Liberal Democratic Party and Komeito.

He beat Masahiko Okada of the Japanese Communist Party, who also ran.

[3] In 2010 he ran[4] and was elected to the House of Councillors as a member of Your Party for the Kanagawa at-large district, defeating Keiko Chiba, who was then the Justice Minister in the DPJ government of Naoto Kan.[5] In 2011 it was reported that he was the third-wealthiest member of the upper house, with assets of ¥258.7 million.

In the 2016 regular election, he was re-elected from Kanagawa as an independent with LDP endorsement with the fourth-most votes behind candidates from LDP, DP and Kōmeitō,[9] and – Kanagawa's fourth seat was the last prefectural district seat countrywide to be called on election night – received retroactive nomination (tsuika kōnin) from the LDP, in effect giving the party two seats from Kanagawa in one election for the first time.