Final answer:
Case arguments related to civil rights are often based upon the Civil Rights Act of 1964, which banned discrimination in public accommodations, employment, and against other bases such as race and gender.
Step-by-step explanation:
The most significant piece of civil rights legislation in US history that case arguments are commonly based upon is the Civil Rights Act of 1964. This act had wide-ranging effects on society, outlawing discrimination based on race, color, religion, sex, or national origin. It addressed issues like segregation in public accommodations, voter registration requirements, and employment discrimination, and established the Equal Employment Opportunity Commission (EEOC) to enforce these provisions.