He moved to the United States in 1932, and graduated from the University of Southern California with a degree in political science.

During World War II, he worked in the Ministry of Foreign Affairs and as a non-combatant in the Imperial Japanese Navy.

Nikaidō unsuccessfully ran as an opposition candidate for the House of Representatives in the 1942 general election.

During this time, he became acquainted with future political allies Kakuei Tanaka and Takeo Miki.

On August 18, he asked an "emergency question" on Amami's reversion to Japan at a Lower House plenary session.