Pete Buttigieg, the 38-year-old former mayor of South Bend, Ind., also put pressure on Mr. Sanders to disclose more about his health, suggesting on Wednesday that he was not being forthcoming and urging all candidates to “do better.”

“I think we should be transparent, especially living in the Trump era,” Mr. Buttigieg said, in an interview with MSNBC. “We’ve got to do better. I would look to the Obama standard of releasing not just a letter from a doctor, but actual results from a physical. That’s what we’re planning to do, and I think every candidate should hold themselves to that same standard.”

Ms. Gray, in the CNN interview on Wednesday, said the calls for Mr. Sanders to put out more information on his health were reminiscent of “skepticism campaigns that have been run against a lot of different candidates in the past questioning where they’re from — aspects of their lineage.”

She then said, without evidence, that one of Mr. Sanders’s primary opponents, Michael R. Bloomberg, had had several heart attacks. Mr. Bloomberg had a stent inserted in 2000 to clear an artery, according to a letter from his doctor released in December. That letter also said Mr. Bloomberg had been diagnosed in 2018 with atrial fibrillation.

Shortly after making her comments, Ms. Gray walked them back. “I misspoke when I said Bloomberg had a heart attack,” she wrote on Twitter.

In his statement, Mr. Sheekey said Mr. Bloomberg had had two coronary stents placed in 2000, after a positive stress test at his doctor’s office.

“He quickly told the F.A.A., consistent with the rules for any pilot, and this information has been public for years,” he said.