It is about the Civil Rights Act of 1964. The Civil Rights Act of 1964 (Pub.L. 88–352, 78 Stat. 241, enacted July 2, 1964) is a landmark piece of civil rights legislation in the United States that outlawed discrimination based on race, color, religion, sex, or national origin. It ended unequal application of voter registration requirements and racial segregation in schools, at the workplace and by facilities that served the general public (known as "public accommodations").
He's calling on the Senators and Congressmen to not stand in the way of this landmark legislation.
"Come senators, congressmen
Please heed the call
Don't stand in the doorway
Don't block up the hall"