You can read a transcript of Civil Rights Movement veterans talking about Mississippi and the Mississippi Freedom Democratic Party at the Civil Rights Movement Veterans website. The transcript includes a section on the 1965 Congressional challenge.
The primary source document that I posted is from the Mississippi Department of Archives and History Sovereignty Commission Online. For those unacquainted with it, the Sovereignty Commission was the state of Mississippi's very own secret police spying operation, which was actively maintained by the state from 1957 to 1973, to oppose the Civil Rights Movement. The Congressional challenge document was collected as part of the Commission's intelligence efforts. Here's a little background by Yasuhiro Katagiri, who has written a book about the Commission:
While Mississippi had already become the birthplace of the Citizens’ Council in private spheres, with the swearing in of James P. Coleman as the fifty-second governor of the state, an overwhelming mood of defiance to the federal government dominated the 1956 state legislative session, which witnessed the introduction of a parade of bills and resolutions designed to protect Mississippi’s racial customs and its sovereignty. Among them, the legislature expressed its determination to defend Mississippi against the “illegal encroachment” of the federal government by adopting the so-called “interposition resolution.”
With the defiance of the federal government at its height and inspired by the issuance of the interposition resolution, Mississippi lawmakers then turned to creating a tax-supported implementation agency of the resolves expressed in the resolution. Thus, on March 29, 1956, with the blessing of Governor Coleman, the state created the Mississippi State Sovereignty Commission as part of the executive branch of its government “to do and perform any and all acts and things deemed necessary and proper to protect the sovereignty of the State of Mississippi.” With the governor as its chair, the Sovereignty Commission was composed of twelve members: the governor, the lieutenant governor, the House Speaker, the attorney general, two state senators, three state representatives, and three citizens.
Despite the fact that the Sovereignty Commission was soon to be identified as Mississippi’s “segregation watchdog agency,” neither the word “segregation” nor the word “integration” appeared in the carefully crafted bill creating the new state agency. But to be sure, federal “encroachment” was a periphrasis implying “forced racial integration,” and “to protect the sovereignty of the State of Mississippi” from that “encroachment” was a sophisticated roundabout expression of the state’s determination to preserve and protect the racial segregation in Mississippi. With the aura of sophistication and respectability emanating from the word “sovereignty,” the state agency, for all practical purposes, was expected to maintain segregation and to wreck the National Association for the Advancement of Colored People (NAACP) and other civil rights organizations in Mississippi.
Since its inception in 1956 until its practical demise in the late 1960s, the Sovereignty Commission had consisted of two main departments, one concerned with public relations and the other with investigation. Under the Coleman administration, by the fall of 1957, more than 200,000 pamphlets and other forms of direct mail had been sent to newspaper editors, television stations, and state lawmakers above the Mason-Dixon Line by the state agency to “educate” the nation on Mississippi’s race relations. Meanwhile, the Sovereignty Commission deployed its paid and unpaid black informants throughout the state to keep the NAACP and the Mississippi Progressive Voters’ League under surveillance. Thanks to the eyes and ears of these informants and its own investigators, by the summer of 1959, the Sovereignty Commission had accumulated over 4,000 index cards and several hundred investigative files containing a hodgepodge of allegations, rumors, and bizarre details, many of them baseless, revealing the hysterical state of Mississippi’s white leaders.
(Whole thing.)