Lt. Gov. John Fetterman of Pennsylvania, the Democratic nominee in what will be one of the hardest-fought Senate contests in the nation, has a heart condition called cardiomyopathy and appeared to have left other heart issues untreated for years, his doctor disclosed in a statement on Friday.Mr. Fetterman, who suffered a stroke days before the Democratic primary last month, had a pacemaker and defibrillator implanted on the day of the primary, which his campaign at the time described as a standard procedure that would address “the underlying cause of his stroke, atrial fibrillation.” His campaign offered few other details about his condition in the days that followed, but doctors questioned the campaign’s characterization of the use of a defibrillator, noting that they are not typically used for atrial fibrillation, and are more often used for conditions like cardiomyopathy — a weakened heart muscle.“Yesterday I talked to John about how, while afib was the cause of his stroke, he also has a condition called cardiomyopathy,” Ramesh R. Chandra, his doctor, wrote in a note. “The prognosis I can give for John’s heart is this: If he takes his medications, eats healthy, and exercises, he’ll be fine. If he does what I’ve told him, and I do believe that he is taking his recovery and his health very seriously this time, he should be able to campaign and serve in the U.S. Senate without a problem.”Cardiomyopathy “is a disease of the heart muscle that makes it harder for the heart to pump blood to the rest of the body,” according to the Mayo Clinic. “Cardiomyopathy can lead to heart failure.”Dr. Chandra said the defibrillator and pacemaker appeared to be “working perfectly and he is doing well.”Dr. Chandra also wrote that when Mr. Fetterman was diagnosed with atrial fibrillation and a decreased heart pump in 2017, he was prescribed medicine, lifestyle changes and follow-up appointments, but he “did not go to any doctor for 5 years and did not continue taking his medications.”“Like so many others, and so many men in particular, I avoided going to the doctor, even though I knew I didn’t feel well,” Mr. Fetterman said in a statement. “As a result, I almost died. I want to encourage others to not make the same mistake.”Dr. Chandra is Mr. Fetterman’s cardiologist, but after the stroke he was initially treated by other doctors at Lancaster General Hospital. They have not been made available for questions.Former Vice President Dick Cheney had a defibrillator implanted in 2001. He finished two terms in the White House, including a hard-fought re-election campaign in 2004. “Doctors have told me I need to continue to rest, eat healthy, exercise, and focus on my recovery, and that’s exactly what I’m doing,” Mr. Fetterman said. “It will take some more time to get back on the campaign trail like I was in the lead-up to the primary. It’s frustrating — all the more so because this is my own fault — but bear with me, I need a little more time. I’m not quite back to 100 percent yet, but I’m getting closer every day.”When he does return to the campaign trail, it appears his Republican opponent will be Dr. Mehmet Oz, the celebrity television physician. With a statewide recount still underway on Friday in the Republican Senate primary and no official race call, David McCormick conceded the race to Dr. Oz.Ed Rendell, a Democratic former governor of the state and a former chairman of the Democratic National Committee, said in an interview on Friday that he had no qualms about Mr. Fetterman’s fitness to serve. He downplayed how much Mr. Fetterman’s health would weigh on the minds of voters, saying that he did not think it would be an issue.“When I was governor, the Republicans used to say I was one cheese steak away from having a heart attack, and I never did,” said Mr. Rendell. Nancy Patton Mills, the chairwoman of the Pennsylvania Democrats, did not immediately respond to a request for comment on Friday. The Democratic Senatorial Campaign Committee did not immediately comment on Friday.Mr. Fetterman has been off the campaign trail since his stroke and has occasionally released brief videos since then. In a sign that Mr. Fetterman was moving back toward some political engagement, Senator Bob Casey, Democrat of Pennsylvania, wrote on Twitter that he’d had a “virtual double date” with Mr. Fetterman and his wife, Gisele Barreto Fetterman, earlier Friday afternoon.“Looking forward to many more on the campaign trail this summer!” Mr. Casey wrote.Gina Kolata