What is the origin of the phrase "useful idiot" in a political context?

The Webpage "Word Histories" gives "useful idiot" as a development of "useful innocent" and suggests that its origins are in 1948 Italian, with its English coinage the same year:

Useful idiot originated as a translation from an Italian expression (itself probably a translation from Serbo-Croat).

From the San Francisco Examiner (San Francisco, California) of Tuesday 6th April 1948:

Italy Threatens To Call Off Vote

Milan, April 5.—(INS)—Italian Interior Minister Mario Scelba 5 announced tonight that the Italian elections will be called off “if individual liberty is endangered.” Scelba said he favored reorganization of the Italian police to enable policemen to guarantee freedom of the ballot. He called Socialist Leader Pietro Nenni 6, who is co-operating with the Communists, the “No. 1 useful idiot assisting Communist aspirations to control Italy.”

The suggested Italian origin of the phrase (noted in Greybeard's interesting and helpful answer, is corroborated by another California newspaper article from 1948. From "Useful Idiots," in the San Bernardino [California] Sun (July 17, 1948):

In Czechoslovakia, for instance, the Communists had less than one-third of the popular vote at the last free elections, not enough to enable them to rule the country alone. They ruled by persuading enough Socialists and others to form a popular front government with them.

Then, less than four months ago, they staged a coup to throw their allies overboard and make sure that the people would never have a chance to vote the Communists out of office.

...

Such fellow-traveling allies are called "useful idiots" by an Italian publication. Like the [Henry] Wallace followers in the United States, they have ignored the record of the last 12 years. Wherever the Communists have had the power, from Madrid in the Spanish civil war to Czechoslovakia today, they have persecuted or killed the useful idiots who refused to accept absolute dictatorship.

But there are fewer useful idiots in Europe than there were. That is why the Communists have so far failed in Italy and France. And that is why they are now considering a change of strategy.

From a fairly early (post-1948) date, the association of "useful idiot" with the notion of being a dupe of the Communist Party seems to have engendered the popular idea that the term originated (in Russian) with Lenin.

One early instance of this explicit connection appears an unidentified item in in Italy Today 1951: The Yearly Review of Italian Contemporary Life (1951) [combined snippets]:

Finally, we have a category of intellectuals who profess to be alien to Communism but support it by associating the Western world only with the idea of freedom, and the Communist world, instead with that of social justice, and therefore advocate an agreement, a conciliation, a fusion of the two worlds. These intellectuals apparently ignore the fact that idea and practice of justice are banned from all countries under Communist rule, but very well serve the cause of Communism as «useful idiots» as Lenin called them.

Several decades later, the connection between Lenin and the term "useful idiot" in right-wing thought was so thoroughly established that many writers considered the attribution a settled fact. Thus, for example, William J. Bennett, in America: The Last Best Hope, volume 2, From a World at War to the Triumph of Freedom (2008) writes (without providing a citation to the source of his information) as follows:

"Useful idiots of the West."

"Useful idiot" was the term Lenin had used for credulous Western businessmen like Armand Hammer who helped build up the Soviet Communist state . "How will we hang the Capitalists," asked one of Lenin's Bolshevik comrades, "we don't have enough rope!" "They will sell it to us—on credit," Lenin famously replied.

Bennett may be offering two spurious Lenin quotes for the price of one here. According to Paul Boller & John George, They Never Said It: A Book of Fake Quotes, Misquotes, and Misleading Attributions (1989):

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 the 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."

And (also from Boller & George):

"The capitalists will sell us the rope with which to hang them."

There may be truth in the much-quoted remark that Lenin is alleged to have made about the capitalists' eagerness to sell their goods (the profit motive is, after all, unideological), but it is almost certainly a fake. Lenin was supposed to have made his observation to Grigori Zinoviev, not long after a meeting of the Politburo in the early 1920s, but there id no evidence that he ever did. Experts on the Soviet Union reject the rope quote as spurious.

Conclusion

The term "useful idiot" in a political sense seems to have reached the English-speaking world from Italy as the translation of an Italian term that that was in use among politicians and commentators in that country in 1948 and began to appear (in translation) in English-language newspapers that same year.

I am unaware of any published matches for the term in the relevant sense prior to the April 6, 1948, instance from the San Francisco Examiner cited in Greybeard's answer. The popular notion that the term goes back to Vladimir Lenin in the early 1920s has persisted despite the utter lack of documentation to substantiate that claim.

Refreshingly, William Safire, Safire's Political Dictionary, second edition (2008) declines to jump aboard the "useful idiot" sealed train. Safire reports that it was he who sought information in 1987 about the connection between "useful idiot" and Lenin from the senior reference librarian at the Library of Congress (quoted in Boller & George's book) and was informed that "the phrase has not been found in any of his writings." Further, Safire notes,

Neither Tass, the Russian news agency, nor Communist Party headquarters in New York City can offer the source.

William J. Bennett could learn a thing or two from Safire about exercising due diligence before attributing familiar quotations to a specific (and famous) source in a formal publication. "Trust and don't bother to verify" isn't a method that his old boss would have approved of, I suspect.