Sanders spokeswoman says she misspoke after distorting Bloomberg's health record

Sanders spokeswoman Briahna Joy Gray later said she misspoke when she said Bloomberg had experienced heart attacks in the past.

Sanders' health has been an issue in his campaign since the 78-year-old suffered a heart attack last fall. He released several letters from physicians late last year and said at a CNN town hall Tuesday night that it's unlikely his campaign will release additional information about his health.

Gray told CNN's John Berman on "New Day" Wednesday that questions about the senator's fitness for office are unfair.

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"What you're seeing right now is really reminiscent of some of the kind of smear, kind 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, et cetera, et cetera," Gray said. "It's really telling given that none of the same concern is being demonstrated for Michael Bloomberg, who is the same age as Bernie Sanders, who has suffered heart attacks in the past."

Bloomberg, who is also 78, underwent a coronary stent placement in 2000 for a blocked artery, but that does not mean he had a heart attack.

Kevin Sheekey, Bloomberg's campaign manager, said Gray's reference to Bloomberg having suffered heart attacks is "an absolute lie."

"Facts matter. This isn't the way to defeat Donald Trump in November," he said in a statement.

Gray walked back her comment about Bloomberg on Twitter later Wednesday morning.

"I mispoke when I said Bloomberg had a heart attack. Rather, he underwent the same stent procedure as Bernie. Bernie released 3 detailed medical reports in December — just like the other candidates," Gray tweeted. Sanders had two stents placed in a blocked coronary artery following his heart attack.

Set to face off on debate stage

The back and forth between the two candidates' campaigns come as Bloomberg is set to make his debut on the Democratic debate stage later Wednesday. Sanders, as the current front-runner in national polls, has accused Bloomberg of buying his way to the nomination by spending millions of his own money on his campaign.

Both candidates -- either of whom would be the oldest president in US history if there were to be elected -- have attempted to assuage voters that they are fit for office.

Last fall, Sanders was rushed to an urgent care facility after experiencing chest pains and was off the campaign trail for about two weeks. Nearly three months later, he released three letters from physicians attesting to his good health.

During the CNN town hall, Sanders argued that his campaign has released "quite as much as any other candidate has" and that the letters, including one from the attending physician at the US Capitol, amounted to a "detailed report."

Asked if he would release more details, the Vermont senator said, "I don't think we will."

Previously, Sanders had pledged in September, before he suffered the heart attack, to release his medical records before the first votes are cast. After his heart attack, Sanders said voters deserve to have "full disclosure" about a candidate's health.

"The people do have a right to know about the health of a senator, somebody who's running for president of the United States — full disclosure," Sanders said in an interview in October with CNN Chief Medical Correspondent Dr. Sanjay Gupta.

In December, Bloomberg's campaign released a letter from the candidate's doctor that said he is in "outstanding health" with no medical conditions that would prevent him from serving as president. Since the stent placement, Bloomberg has undergone annual cardiac stress testing and last year's test was "normal and demonstrated excellent exercise tolerance," the letter said.

The letter stated that the former mayor takes a blood thinner after developing atrial fibrillation, which is an irregular heartbeat that can lead to blood clots, stroke, heart failure and other heart-related complications. At least 2.7 million Americans are living with atrial fibrillation, for which the common treatment is medication, according to the American Heart Association.

This story has been updated to include additional comment from Gray and background information.