The document that served as the foundation for civil rights legislation in the 20th century was the Constitution, specifically its amendments, particularly the 14th and 15th Amendments. The 14th Amendment, ratified in 1868, guarantees equal protection under the law and was instrumental in challenging racial segregation and discrimination. The 15th Amendment, ratified in 1870, aimed to secure voting rights regardless of race. The Constitution's interpretations by the courts and subsequent civil rights legislation, such as the Civil Rights Act of 1964 and the Voting Rights Act of 1965, were built upon the ideals enshrined in these amendments, solidifying the legal basis for civil rights advancements.