The influential politician known for their role in expanding civil rights legislation is Lyndon B. Johnson. As President of the United States from 1963 to 1969, he championed landmark civil rights laws, including the Civil Rights Act of 1964 and the Voting Rights Act of 1965. Johnson's commitment to ending racial segregation and discrimination marked a significant turning point in American history. His "Great Society" programs aimed at addressing poverty and racial injustice highlight his dedication to social reform, making him a pivotal figure in the civil rights movement.