Final answer:
The HMO Act of 1973 was mainly responsible for giving states the authority to enroll Medicaid recipients into managed care.
Step-by-step explanation:
The legislation mainly responsible for giving states the authority to enroll a large number of Medicaid recipients into managed care is the HMO Act of 1973. The HMO Act encouraged the creation of Health Maintenance Organizations (HMOs), which are managed care organizations that provide healthcare services to enrolled members under a fixed monthly payment. This legislation paved the way for the expansion of managed care in Medicaid, allowing states to contract with HMOs to cover Medicaid recipients.