Full Idea
Between the capitalist and communist society lies the revolutionary transformation of the one into the other. Corresponding to this is a political transition period in which the state can be nothing but the revolutionary dictatorship of the proletariat.
Gist of Idea
In moving from capitalism to communism a revolutionary dictatorship of the proletariat is needed
Source
Karl Marx (Critique of the Gotha Program [1875], IV)
A Reaction
This hugely influential idea was catastrophic for the twentieth century, because the leaders of the proletarian dictatorship adored and abused the power, and wouldn't give it up for some feeble next stage.
Book Reference
Marx,Karl: 'Selected Writings', ed/tr. Simon,Lawrence H. [Hackett 1994], p.328