Kainulainen was from June to August 2017 the oldest surviving MP together with Pentti Poutanen, who was born on the same day.

[5] Kainulainen focused on the activities in the Finnish Democratic Youth League (SDNL, Suomen demokraattinen nuorisoliitto).

Kainulainen resigned from the SDNL in the spring of 1955 after failing to make the union more efficient, and he went to Moscow for studies.

He strongly condemned any form of revisionism, which represented the majority party led by Aarne Saarinen.

[8] Kainulainen was a key figure in the process of turning it into a joint publication of the entire opposition in the autumn of 1968.

At the rebellion meeting in the Koitto House in Helsinki, Kainulainen supported the establishment of a new ideologically orthodox communist party, but was in a minority with his opinions.

In April 1969, Kainulainen was elected as the executive committee secretary of the Communist Land Advisory Board, which was founded by the opposition.

[11] In the 1970s Kainulainen continued his principled struggle against revisionism and he was alongside Taisto Sinisalo and Urho Jokinen one of the most visible figures in the taistoism movement.

[13] Under the leadership of Kainulainen, the SKP's Uusimaa district was firmly in the hands of the opposition and represented the left wing of the taistoism group.

In October 1985, the SKP sacked the district organizations of the opposition, including Uusimaa, under the leadership of Arvo Aalto.

Kainulainen was KTP's most prominent politician and supporters of the party have been called, with regard to his name, the Kainus (kainuslaiset).