The ostensible basis for Miller's "The Crucible" was the persecution of eccentrics or anyone who was different as witches in Salem and other communities in the early American colonies. A book about the topic came out in 1949 which stirred more interest in the events surrounding the witch trials.
But the topics and the themes of the play are also relevant when it comes to McCarthy and his Un-American Activities Committee which was formed to try and root out communist activism in the United States but was also used to attack anyone or anything that was not in-line with what McCarthy and other conservative politicians thought was appropriate.
In both situations, the willingness of people in power to use their power to persecute those that were different than them or those who opposed them in any way is the subject of Miller's criticism. Miller himself had many friends who were blacklisted and persecuted and sometimes even prosecuted because of what McCarthy or his cronies considered un-American behavior.