Nineteenth-century federal and state obscenity laws provided officials with extensive censorship powers to ban "obscene" materials, ranging from literature to birth control information and devices. Promoted by Anthony Comstock, the obscenity laws suppressed sexual knowledge in the name of protecting traditional sexual morality. Federal and state courts routinely upheld the laws, even as social behaviors challenged cultural norms and as public interest in sexual information grew. In the late 1920s New York City lawyer Morris L. Ernst and his Greenbaum, Wolff & Ernst colleagues began a sustained campaign in the city, state, and federal courts to challenge the obscenity laws. They defended writers, publishers, birth control activists, sex educators, and sex researchers in winning a series of cases to make the laws more compatible with changing cultural practices and to permit the public a wider marketplace of information about sex and reproductive control.

Mary Ware Dennett, a suffragist and birth control pioneer, wrote a sex education manual, The Sex Side of Life, published in 1918. Popular among parents and youth educators in the YMCA and respected for its medical accuracy, the Postal Service declared the pamphlet obscene. Dennett's nonjudgmental treatment of masturbation and her clinical description of sexual intercourse alarmed her critics. In 1929 U.S. officials indicted Dennett for using the mails to distribute her "obscene" pamphlet. Morris Ernst and Alexander Lindey won a landmark decision overturning her lower court conviction in the Second Circuit Court of Appeals. The United States v. Dennett (39 F.2d 564) decision provided a path forward for subsequent obscenity jurisprudence by securing protection for serious sex instruction materials whose purposes were not "clearly indecent." The court accepted the argument that the work's medical and scientific value was greater than its potential harms to vulnerable readers.

Famous as the leading birth control activist in England, Marie Stopes first came to public attention for her widely popular book Married Love (1918). U.S. Customs officials banned importation of the book in the United States in 1922, libeling it as obscene for its attention to the importance of female sexual pleasure. The book's opponents warned that it would undermine marriages. Morris Ernst and Alexander Lindey, hoping to use the United States v. Dennett decision and revisions in the Tariff Act of 1930, challenged the Customs Service's censorship authority on behalf of Stopes and her publishers. In 1931 federal court judge John M. Woolsey held the book not obscene. In a follow-up case Judge Woolsey also held that Stopes's 1923 book Contraception (a history of contraceptive practices) was also not obscene and could be admitted to the United States.

Radclyffe Hall wrote the most famous English-language lesbian-themed novel of the first half of the twentieth century, The Well of Loneliness, in 1928. The novel faced immediate attacks for being obscene, and the London publisher, Jonathan Cape, was twice convicted in English courts in highly publicized trials. A newly formed U.S. publishing house, Covici-Friede, seized on the book's notoriety and published it in the United States, knowing that the Vice Society's John Sumner would bring obscenity charges. Morris Ernst and Alexander Lindey lost the first trial against Covici-Friede when the presiding magistrate denounced lesbianism as an obscene subject. They secured a victory in the New York Court of Special Sessions in 1929, succeeded in getting the Customs ban on the book lifted, and began their campaign against literary censorship.

In 1930 Morris Ernst, Alexander Lindey, and their journalist and publisher allies planned to wage a fight with the Vice Society's John Sumner. They agreed to a censorship battle around Simon & Schuster's publication of Arthur Schnitzler's novel Casanova's Homecoming, knowing that Sumner would attack the novel. Sumner proved more effective in censoring tawdry works than they imagined, and he gained momentum with his successful attacks on Nathan Asch's Pay Day and another Arthur Schnitzler work, Hands Around. New York State judges proved willing to uphold the obscenity statutes. Ernst and Lindey succeeded in their defense of Casanova's Homecoming, but jurists and literary authorities chafed at the "vomit school of literature" that was polluting the literary marketplace, and New York's highest court upheld lower court bans on obscene works.

Notorious for its frank treatment of bodily functions, sexual reveries, masturbation, adultery, and blasphemy, early episodes of James Joyce's Ulysses published in The Little Review were censored by U.S. Postal authorities and the magazine publishers were convicted for distributing obscene literature in 1920. Published in novel form in 1922 by Paris bookstore owner Sylvia Beach, Joyce's novel was still banned throughout the English-speaking world in 1933. Regarded as the great literary masterpiece of the modern era, the censorship of Ulysses outraged literary opinion. Morris Ernst and Alexander Lindey together with Random House publisher Bennett Cerf challenged the U.S. Customs ban on Ulysses. Working closely with U.S. Attorneys, they brought the case before Judge John M. Woolsey, who cleared the book in 1933. The U.S. Court of Appeals for the Second Circuit upheld the ruling in 1934.

In 1915 police raided Margaret Sanger's first birth control clinic; she and her sister, Ethel Byrne, were convicted under the New York State obscenity statutes. In People v. Sanger (1918) the New York Appellate Court upheld Sanger's conviction but provided a "physicians' exception" ruling, permitting doctors to prescribe contraception. When police raided Sanger and Dr. Hannah Stone's Birth Control Clinical Research Bureau in Manhattan in 1929, Morris Ernst successfully defended the clinic's doctors and nurses in People v. Sideri (1929). Ernst and Harriet Pilpel later orchestrated a federal court challenge to U.S. Custom's prohibitions on importing contraceptive devices. They won a landmark case in United States v. One Package of Japanese Pessaries (1936). Ernst and Pilpel then expanded the ruling to protect research scientists' importing contraceptive information in United States v. Norman E. Himes (1938).

Morris Ernst and Alfred Kinsey of Indiana University forged a lawyer-client relationship just as Kinsey was about to publish his first major study, Sexual Behavior in the Human Male in 1948. Ernst recognized the value of Kinsey's work for his own "sex enlightenment" project, and with David Loth wrote a short book promoting Kinsey's findings, American Sexual Behavior and the Kinsey Report. He also urged two judges in different jurisdictions, Curtis Bok and Jerome Frank, that Kinsey's findings would support anti-censorship arguments in the obscenity cases before them. Ernst and Harriet Pilpel also took on the defense of Kinsey and the Institute for Sex Research in a Customs seizure case, United States v. 31 Photographs (1957), decided in their favor by Judge Edward Palmieri shortly after the U.S. Supreme Court made its landmark obscenity law ruling in Roth v. United States, also in 1957.

For Morris Ernst, Alexander Lindey, and Harriet Pilpel, the value of sexual knowledge and the right of public access to serious materials dealing with sexual matters outweighed the potential harms of sexual arousal. Ernst and his colleagues understood the connections between reading about sex, having greater knowledge of it, being able to control reproduction, and taking pleasure in sex. Their anti-censorship campaign achieved free speech advances in the realm of obscenity law, paralleling developments in First Amendment jurisprudence in the same era. They paved the way, legally, to the sexual revolutions of the 1960s and beyond by making obscenity laws less capacious and judges more willing to protect sexual speech, making Ernst's project of sex enlightenment part of the larger free speech, civil liberties history.

By the late 1930s the ACLU was accused of a being a communist front organization by Congressman Martin Dies. ACLU leaders combated Dies's attacks but were also increasingly divided about the perceived threat of communists within the organization. Following the August 1939 Nazi-Soviet Pact and in tacit cooperation with Dies, the ACLU's leadership purged known members of the Communist Party from its executive committee. Morris Ernst took a strong anticommunist position in these internecine battles and began to develop a long-term cooperative relationship with the FBI director, J. Edgar Hoover, promoting Hoover's anticommunist investigations and defending him against attacks, including from the ACLU itself. Although he remained on the ACLU's executive committee until 1954, Ernst had retreated from his strong civil liberties commitments and became a committed anticommunist.