Testifying for more than three hours, Jackson Reffitt, now 19, told the jury how his father had become more distant and severe in his beliefs in 2016, the same year Donald J. Trump was elected president. Father and son, he said, did not see eye-to-eye on politics.

“I was moderately left and my father was moderately right,” the younger Mr. Reffitt said, adding that during that election year, “we both went further in our own direction.”

Jackson Reffitt also said his father was a member of the Texas Three Percenters, a state militia group closely linked to the gun rights movement. Guy Reffitt flew a flag outside the family’s home in Wylie, Texas, emblazoned with a Three Percenters’ logo. His son told the jury that he often went about his business with a .40-caliber pistol on his hip.

Things became more tense between the father and son in December 2020, Jackson Reffitt said, as Mr. Trump was undertaking multiple, overlapping schemes to reverse his election defeat. Much of the conflict played out on a family group chat, several messages of which were shown to the jury Thursday.