Ernst's support of the FBI was public, but he performed numerous favors on behalf of the Bureau secretly. I'm summing up information from various sources I've read that would take some time to track down, but I can cite a couple here that I have handy. Athan Theoharis comments a bit on some FBI files mentioning Ernst's relationship to the Bureau in his biography of J. Edgar Hoover,
The Boss, p. 238f and 302-305; on 238 he calls Ernst "Hoover's mole in the liberal community". One website summarizes the genesis of Ernst's work for the FBI in the late 1930s:
Marilyn Bardsley, "J. Edgar Hoover"
In 1938 when Hoover was forty-three, the most important person in his life, his mother Annie Hoover, died. . .Incredibly enough, Morris Ernst, the head of the American Civil Liberties Union, became a staunch supporter of Hoover and his FBI during this period. A deal was struck between the two men whereby the ACLU promised to purge itself of any known Communists and Hoover would keep Congressman Martin Dies’ House Un-American Activities Committee away from the ACLU. . .