The former chief of staff to Senator John Fetterman, Democrat of Pennsylvania, was so alarmed with his ex-boss’s erratic behavior last year that he wrote a lengthy letter to his doctor warning that the senator was spiraling out of control and that his mental health issues could cost him his life.

“I’m worried that if John stays on his current trajectory he won’t be with us for much longer,” Adam Jentleson, the former chief of staff, wrote on May 20 to a doctor who had treated Mr. Fetterman at Walter Reed National Military Medical Center.

Mr. Fetterman’s behavior, according to former aides who are still connected to his diminishing circle, is still at times a cause of concern. Other former members of his staff, speaking on the condition of anonymity, report that their colleagues sometimes were frightened to be in the senator’s presence, if he was in an amped-up mood.

They have also long been warned never to get in a car if Mr. Fetterman is behind the wheel because of his dangerous driving habits. His volatile and concerning behavior, which aides noticed last year was taking a turn for the worse, has only increased since the election, people who have spent time with him said. That has coincided with a period when his politics have become more conservative, as he has watched his home state of Pennsylvania swing for Mr. Trump.

“He does not see his doctors,” Mr. Jentleson wrote last year to the medical director who oversaw his 2023 hospitalization for mental health issues. “I am not sure when he last saw a cardiologist, but I don’t think he’s seen one since he was released. He long ago ordered us to stop putting regular drop-bys with Dr. Monahan on his schedule, despite the fact that he had agreed to those as part of the plan.” Dr. Brian P. Monahan is the Navy doctor who has served for nearly 15 years as the on-site physician in the Capitol.

Mr. Jentleson’s letter, which was obtained by The New York Times, was first reported by New York magazine.