One of the many hoaxes of the 20th century was the idea that Walter Reuther, a prominent labor activist and head of the United Auto Workers had written a secret letter which detailed a plan for a "Soviet America"
Reuther and his brother had had positive feelings toward the Soviet Union earlier in their lives, but the supposed letter went much further than that, presenting the brothers as working on behalf of Soviet interests.
A couple things jump out at me about this hoax.
If you're looking for an authoritative source, finding something in the Congressional Record might seem to do the trick. But sadly, no.
This letter is a good example of that. As noted in They Never Said That:
The Reuther quote was a fairly old hoax. Time and again anti-labor Congressmen have inserted into the Congressional Record the letter from which it was taken, and it was a standard quote in right-wing publications in the fifties and sixties. But on September 22, 1958, Democratic Senator John L. McClellan of Arkansas revealed that not only were there three different versions of the Reuther letter in circulation, but also that three different versions of it had appeared in the Congressional Record.