Stalin’s purge eliminated over half of all party members as a class.

Evidence

It took Stalin about two years, from 1936 to 1938, to rid himself of the whole administrative and military aristocracy of the Soviet society, nearly all offices, factories, economic and cultural bodies, government, party, and military bureaus came into new hands, when “nearly half the administrative personnel, party and nonparty, had been swept out,” and more than 50 per cent of all party members and “at least eight million more” were liquidated. — Hannah Arendt, The Origins of Totalitarianism, 766