He was noted in his early political career as a champion of universal suffrage, social welfare, labor unions, women's rights and Pan-Asianism.

[1] He returned to graduate from Waseda with a degree in Colonial Studies, and with the assistance of Ōkuma Shigenobu obtained a post there as a teacher.

[2] Nagai ran for a seat in the Lower House of the Diet during the 1917 General Election from the Kanazawa district, but was defeated by the Rikken Seiyūkai candidate Nakahashi Tokugōrō by only 203 votes.

During his later career, Nagai became increasingly critical of what he perceived to be the racist and imperialistic designs of United States and United Kingdom on Asia, writing essays and issuing speeches on the "White Peril" and on the mission of Japan to protect China from the western powers and to liberate Asia from its European masters.

In a speech in 1943, he promoted the concept of hakkō ichiu as meaning the "universal brotherhood" of mankind, rather than imperial domination of the world by Japan.