" the role of Zionists in the Bolshevik/Communist revolutions"
Veritas, to be clear, Zionists did not play a major role in the Russian  Bolshevik revolution-- they were opposing ideologies, which is what Churchill meant by the struggle for Jewish hearts.  Jewish activists had to choose between the Zionist dream of a happy Jewish homeland in Palestine or the  Marxist vision of heaven on Earth following a Communist/Socialist/Anarchist  revolution in Russia.
Indeed, many Russian  Jews, particular those of a more secular background, chose Bolshevism.  Many others, however, were leaders in more moderate, democratic socialist organizations in Russia, as the Mensheviks. It's ironic that while many of the leading Bolsheviks were Jewish, Stalinist Russia became cruelly anti-Semitic.
Additionally, many of the early Zionist settlers in Palestine were also socialist, resulting in the famous kibbutz communities.