Mayfield, a lawyer, was one of three people allegedly involved in an alleged plot to photograph Sen. Thad Cochran's wife, who is in a nursing home and reportedly has Alzheimer's disease. Cochran, R-Miss., defeated a Tea Party challenger, state Sen. Chris McDaniel, in Tuesday's runoff primary.
The Jackson Clarion-Ledger said sources reported Mayfield appeared to have taken his own life.
Mayfield and two other men were arrested in May. A prosecutor said Mayfield was familiar with the nursing home where Cochran's wife lived because his mother had been a resident and was able to give directions on how to get in.
Grant Sowell, a leader in the Tupelo Tea Party, said Mayfield, who was free on bail, changed after his arrest, appearing to be "cut off from the world." He said when he called Mayfield the lawyer told him investigators might have placed a tap on his phone.
"Nobody should have died over this," Sowell told Politico. "It's just an election. I'm heartbroken by it."
Gov. Phil Bryant paid tribute to Mayfield.
"Deborah and are saddened to hear of the loss of Mark Mayfield," Bryant said in a statement. "He was a long-time friend, and he will be missed. Our prayers go out to his family in this tragic moment."