In the trade union movement, Horowitz served as the assistant to the civil rights leader Bayard Rustin from 1964 to 1973 when he was president of the A. Philip Randolph Institute. She later became the political director of the American Federation of Teachers and assistant to its president, Albert Shanker, who had enormous influence in the union movement and politics. William Galston of the Brookings Institution once described Horowitz's role. Referring to the 1973 Woody Allen movie “Sleeper”—in which “a thawing Woody is informed that most of civilization was destroyed when a man named Albert Shanker got the Bomb” – Galston joked, “Well, if Al Shanker had gotten the bomb, Rachelle would have been in charge of target selection.”