[1] During the interwar period, Kopecký was a member of the underground communist cell in the Karlín area of Prague, along with future party leaders Klement Gottwald and Rudolf Slánský.

Liquidation of anti-Semitism cannot be allowed to cause harm to the national and Slavic character of the future Czechoslovak Republic.

[8] Kopecký was noted for his antisemitic statements, criticizing Jews for Zionism and cosmopolitanism; he also stage-managed the Slánský trial.

According to Jewish historian Michal Frankl, Kopecký "distinguished himself with antisemitic diatribes," criticizing the presence of Jews in politics and attacking Zionism and cosmopolitanism.

[9][10] In 1945, he accused the "Jewish super-rich like Petschek, Weinmann, Rothschild, Gutman" of "blood-sucking" and argued that wealthy Jews could not live in the people's democracy.

[2] According to Kopecký, this demonstrated that the party was not taking the anti-cosmopolitan campaign seriously enough and was underestimating the "very serious danger" posed by Zionism.

Kopecký adhered to a Stalinist line, trying to keep the party in the positions of the Gottwald period under the new conditions.

In 1990, his ashes were moved to Olšany Cemetery, together with those of about 20 other communist leaders which had also originally been placed in the Jan Žižka National Monument.