Mr. Kirk was 18 when he started the conservative youth organization Turning Point USA, taking a sledgehammer to mainstream Republican hierarchies and liberal politics. His group was credited by Mr. Trump as crucial to his election victory in 2024.

Turning Point has spread to more than 850 college campuses, runs vast voting drives and has become one of the most powerful forces in galvanizing a new generation of conservatives.

Mr. Kirk’s views on social issues, including marriage, abortion and gender, were steeped in his conservative interpretation of the Bible. Before his death, many saw those views as abhorrent. He called the Rev. Dr. Martin Luther King Jr. “awful” and “not a good person.” He described the Civil Rights Act as a “huge mistake” and George Floyd as a “scumbag.” He said that Islam “is not compatible with Western civilization,” and accused “Jewish donors” of fueling radicalism by financing “not just colleges — it’s the nonprofits, it’s the movies, it’s Hollywood, it’s all of it.” Democratic women, he said, “want to die alone without children.”

Such statements continue to divide Americans, but with his murder, critics have had to tread lightly.

Mr. Kirk, despite his biblical bent, was also a vocal and committed supporter of Mr. Trump, who has divorced twice and notoriously bragged about how his celebrity let him grab women by their genitals.

Mr. Kirk never ran for office or held any position in either Trump administration, but he said he had visited the White House more than 100 times. He was also an influential supporter of Vice President JD Vance and Defense Secretary Pete Hegseth.

He lived in Scottsdale, Ariz., with his wife, Erika, and their two children. Ms. Kirk took over as chief executive of Turning Point after Mr. Kirk’s death, and is expected to be in the courtroom in Utah as prosecutors detail how her husband’s killing unfolded.