Perhaps it was the man's strong but plain-spoken rebuke outside a Roy Moore rally on the campaign's final night, condemning the Republican candidate's past comments lambasting homosexuality.
Perhaps it was the admission of the man, a peanut farmer, that he too, had harbored some of the same anti-gay feelings.
Perhaps it was his sign, a photograph of his daughter, a lesbian who, he said, had killed herself when she was 23.
Alabama voters will go to the polls Tuesday to choose between Doug Jones, a moderate Democrat who came to prominence helping to prosecute Ku Klux Klan members as a U.S. attorney in the 1990s, and Roy Moore, a far-right conservative and former judge whose candidacy has sharply divided the party he represents and the electorate beyond.
The 74-year-old Mathis, a former county commissioner and state representative in Alabama, said he was speaking out against Moore because of his own experience with his daughter, Patti Sue.
He said that Moore's comments on homosexuality amounted to calling gay people "perverts."
"This is something people need to stop and think about," Mathis said. "You're supposed to uphold the Constitution. The Constitution said all men were created equal. But how is my daughter a pervert just because she's gay?"
Moore, whose politics are sharply tinged by a rigid interpretation of Christianity, has a long track record of speaking harshly about gays. He has said that homosexual conduct "should be illegal," that it is "an inherent evil, and an act so heinous that it defies one's ability to describe it," and that "sodomy is against the laws of nature."
Mathis, who described himself as religious, said that he too shared some of those anti-gay beliefs.
"I said bad things to my daughter myself, which I regret," he said. "But I can't take back what happened to my daughter. Stuff like saying my daughter was a pervert, I'm sure that bothered her."
But he said Moore's thoughts on gay people rang false to him.
"We don't need a person like that representing us in Washington," he said. "That's why I'm here."
He held a sign that noted the accusations that surfaced during the campaign by women who said that Moore made sexual advances toward them when they were teenagers.
"So that makes him a pervert of the worst kind," the sign read. "Please don't vote for Roy Moore!"
Moore has denied these allegations.
Mathis wrote about his daughter in a letter to the Dothan Eagle, a small Alabama newspaper, in 2012.
Born in 1972, she was "a wonderful child" who was "very athletic, tomboyish (I always had to pitch batting practice to her after Dixie Youth practice), very beautiful and smart," he wrote. But after he learned that she was gay from a friend while she was in high school, he confronted her and "said some things to her that still eat on me to this day," he wrote, though he later apologized.
A few years later, she killed herself. Mathis wrote that he found her; she was 23.
"She was tired of being ridiculed and made fun of," Mathis wrote. "She was tired of seeing how a lot of people treat gay people."
He described another moment of regret after his daughter's death, after sitting in a church while a preacher bashed gays.
"I was ashamed of myself for sitting there and not defending Patti," he wrote. "May God have mercy on us all. I only know I miss my daughter Patti very much and I am grateful for having her as my daughter."
Mathis' exact views on gay people were not entirely clear. In the 2012 letter, he writes of taking his daughter to doctors and psychiatrists in the hope of resolving her sexuality at her request. He has run for office as both a Democrat and a Republican.
On Monday, a reporter asked him what he was hoping to accomplish with his protest.
"I had mixed emotions about coming, but somebody needs to speak up," he said. "And if it's all to no avail, so be it. It won't be the first time I've done something to no avail."
The Washington Post's Marwa Eltagouri, Kayla Epstein and Jenna Johnson contributed to this report.