This is a well-known falsification, more often attributed to Lenin.
Here is the part of the relevant entry from _They Never Said It_, by
P. F. Boller, Jr., and John George (p. 76):
Lenin, it is said, once described left-liberals and Social
Democrats as "useful idiots," and for years anti-Communists have
used the phrase to describe Soviet sympathizers in the West,
sometimes suggesting that Lenin himself talked about "useful
idiots of the West." But that expression does not appear in Lenin's
writings. "We get queries on _useful idiots of the West_ all
the time," declared Grant Harris, senior reference librarian at
the LIbrary of Congress in the spring of 1987. "We have not been
able to identify this phrase among his published works."
William C. Waterhouse