Sen. Lindsey Graham's sudden death earlier this month sent President Donald Trump reeling and sunk him into an obsession with his own mortality.Graham, at 71, was nearly a decade younger than the president and had remained visibly active — traveling, campaigning and working — right up until his death, and that contrast has clearly unsettled Trump, reported the Washington Post. In public remarks, he's repeatedly expressed disbelief that someone so energetic could be gone so quickly, telling NBC he "thought he was just going to live forever."“A lot of people can’t do it at all," Trump told “Fox & Friends” on July 13. "A lot of people, when the camera goes on, they’re terrible. I see it all the time. I watch people. Most people aren’t very good, and he did it and he did it with pride and he did it because he loved our country."Privately, according to people familiar with his comments, the 80-year-old Trump has pressed aides on whether Graham's grueling travel schedule — including a return trip from Ukraine shortly before his death — might have played a role, and has turned to genetics and family history for explanations, invoking what he called a "racehorse theory" of inherited strength and stamina.“His father died just about at the same age,” Trump told Newsmax on July 13. The introspection comes at a moment when Trump's own mortality has already been unusually present in his mind. Graham's death fell just before the second anniversary of the assassination attempt in Butler, Pennsylvania, an event Trump says he still revisits by watching footage of it repeatedly, describing his survival as a "miracle."He has also faced more recent threats, including a reported Iranian-linked threat this month that prompted him to switch planes, and an attempted shooting near a White House correspondents' event in the spring.“Because of the position I’m in, I’ve done quite a bit of research into the word ‘assassination,’ Terrible word. And they go after consequential presidents,” Trump told CBS in April, after an attempted shooting at the White House Correspondents’ Association dinner. “I wasn’t worried. I understand life. We live in a crazy world.”Those around Trump describe him as subdued in the days following Graham's death — a notable shift for a president known for near-constant motion and activity.Even so, he hasn't fully stepped back from politics: He's called on the Senate to pass a cryptocurrency bill "in honor of" Graham, despite the senator not having sponsored it.