witch hunt

First recorded in 1885–90 for def. 1; in 1935–40 for def. 2; and in 1945–50 for def. 3; the gerund witch-hunting was first recorded in 1635–40

The atrocities and fallacies of literal, historical witch hunts make for an enduring metaphor to describe persecution of innocents by a torch- and pitchfork-wielding mob.
Historically, people were accused, tried, and punished for the practice of witchcraft in early modern Europe from the late 15th century through the mid-18th century. Many factors contributed to this dark chapter in history, from lack of scientific knowledge, high mortality rates, and natural disasters to social forces including sexism, racism, and ageism.
To understand the figurative use of witch hunt look to the most famous American example of a literal witch hunt, the Salem witch trials of 1692. Various tragedies, from untimely deaths to the loss of livestock, were blamed on "harmful magic." The first three accused in Salem were an enslaved woman, a homeless beggar, and a social outcast. As was common in witch trials, defendants confessed under duress and named confederates. The scope of the trial grew ever wider, with many innocents being convicted and sentenced to death.
The spread of accusations, the implicating of alleged accomplices, and the lack of due process and proper prosecutorial rigor made witch hunt an apt political and social metaphor in the modern era. From Senate subcommittee hearings on pro-German propaganda in the early 20th century to the House Un-American Activities Committee of McCarthyism, witch hunts are now evoked for political investigations characterized by paranoid hysteria, self-preservation, and cynical social machination.
The current use of the expression takes for granted the modern assumption that witchcraft was never a legitimate target of investigation or prosecution. The process of any historical witch hunt was therefore inherently flawed or corrupt, and the target was necessarily innocent. This anachronistic understanding is the crux of the logical fallacy often encountered in modern rhetorical use, where framing an inquiry as a witch hunt is a debate tactic used to assume the innocence of the accused and call into question the motivation and methods of accusers, without examining the substance of the accusation.