Sanders' staffers also plan to emphasize how his health scare demonstrates the need for Medicare for All.

Throughout the Democratic Party, however, insiders and strategists are openly questioning the effect his heart attack would have on his bid for the White House — and on the primary as a whole. Sanders' team is also coming under fire from some journalists and Democrats for not announcing earlier that he suffered a heart attack. His aides have declined to allow reporters to interview Sanders’ doctors.

“It’s one of those things where the cover-up is worse than the crime,” said Andres Ramirez, a Nevada-based Democratic strategist and former vice chairman of the Democratic National Committee’s Hispanic Caucus. “I don’t think anybody would have cared if they said he had a heart attack, got out a few days later, and then everything’s good.”

Instead, he said, “There seemed to be a refusal or hesitance to say, ‘Bernie Sanders had a heart attack.’ … I think it’s less of an issue about his age and more of an issue of, ‘Hey, Bernie, you’re supposed to be the transparent candidate.’”

In an all-staff conference call described to POLITICO by people who listened in, Sanders suggested on Monday that he would use his experience to pivot back to one of his campaign’s central ideas. The same afternoon, he and his wife, Jane, also made a brief appearance leaving his home in Vermont for what appeared to be a brisk walk.

Not one person with this hysterical take can articulate a legitimate risk that three days of privacy (followed by disclosure) over a year before an election presented to the republic. Briahna Joy Gray, Bernie Sanders’ national press secretary

"If there's anything that this event kind of tell us, it is the importance of what our message is in this campaign. And our message is, 'Us, not me.' This has never been about me," Sanders told his team during the call. "It's been about millions of people, including certainly our campaign staff, working together to bring about the kind of fundamental changes this nation absolutely needs."

Sanders’ aides are adamant they handled the health incident appropriately and have taken to social media to defend themselves against criticism, which has come from reporters such as NBC News correspondent Andrea Mitchell and Rolling Stone senior writer Jamil Smith. The aides said they provided several updates on Sanders’ recovery last week, released a statement from his doctors, and made his wife available for questions.

“Not one person with this hysterical take can articulate a legitimate risk that three days of privacy (followed by disclosure) over a year before an election presented to the republic,” Briahna Joy Gray, Sanders’ national press secretary, wrote on Twitter.

Mike Casca, a spokesman for Sanders, told POLITICO that the campaign disclosed that he had a heart attack “when the most accurate and up-to-date summary could be given by doctors."

When Sanders returns to the stump, he will be confronted with a barrage of questions about his age and health — a less-than-ideal position for any candidate, let alone one who often loathes talking about his personal life. Polls have shown for months that many Democratic voters are wary of nominating a candidate in their 70s, and Sanders had slipped to third place in national polling averages before his heart attack.

“Bernie will address it by having a strong debate performance, by being out on the trail, and showing that he’s more energized and has more fighting spirit than ever before,” said Rep. Ro Khanna, Sanders’ campaign co-chair. “I think that’s ultimately what will convince people that he has the fight in him.”

Added a Sanders aide: “As Bernie’s done his whole career by being right on the issues even when no one else was with him, we'll be in ‘show, don't tell’ mode.”

Sanders’ surrogates also expect to deal with questions about his heart attack in part by discussing Medicare for All, his signature issue. Vermont Lt. Gov. David Zuckerman, a surrogate for Sanders, said that because he has good, government-provided health care, his hospitalization will not hurt him financially — whereas many people in the same situation “would be fine one day and bankrupt the next.”

Some of Sanders’ allies also believe he should use his experience to get more personal with voters, something his aides have long pushed him to do in general, and trim his relentless campaign schedule.

“I hope he uses this to share a part of him,” said Khanna. “If he shares that moment of vulnerability and his gratitude for having access to the greatest health care in the world, and his resilience to fight for that for every person … that will draw a human connection with people.”

Sanders’ team has not said when he will return to the campaign trail, but has confirmed that he will participate in the Oct. 15 debate. Minnesota Attorney General Keith Ellison, a top ally to Sanders, said the Vermont senator is nonetheless back to work.

“Bernie’s doing fine,” he said. “He’s working and he’s busy and he’s thinking and he’s planning.”

Political operatives who have worked for presidential candidates who experienced health scares on the campaign trail said transparency is key.

“It’s better to be as upfront as you can as quickly as you can,” said Karen Finney, a former spokeswoman for Hillary Clinton's 2016 campaign, adding that there may have been a medical reason that Sanders’ team did not disclose his heart attack earlier. “Because to some degree, it becomes a trust issue.”

Gina Glantz, the former campaign manager for Bill Bradley’s 2000 presidential bid, said she believes that his treatment for an irregular heartbeat during the election was one of the reasons he lost the New Hampshire primary. She said Sanders has two things going for him that Bradley did not: The first votes will not be cast for another four months. And social media exists now.

“For five weeks, there was virtually no mention of Bill Bradley in the press, and we didn’t have the tools to keep him alive in the voters’ minds,” she said.

If Democrats nominate an older candidate — whether it's Sanders, Joe Biden or Warren — some party insiders worry that they will cede a generational contrast that they might otherwise have made with President Donald Trump, the oldest person ever to take office. But Trump’s age — he is now 73 — is also seen as neutralizing the effect of an older Democratic nominee.

Biden, Sanders and Warren have all taken steps during the campaign to project an image of vigor — Biden and Warren by running at parades and public events, and Sanders with his frenetic schedule. Sanders also recently played a softball game with his aides and others, and frequently shoots hoops on the campaign trail.

If his heart attack does rattle Sanders' supporters, several Democratic operatives said it's likely to benefit his friend and fellow progressive Warren. But the effect may be more limited than if Sanders were competing against a much younger Democrat in the top tier.

Biden, Sanders and Warren are all septuagenarians who enjoy high name ID, and Democrats like them in spite of — or, in some cases, because of — their advanced age. Some of Warren’s allies also believe that Sanders being in the primary actually benefits her by providing a shield against the more moderate wing of the party as well as other opponents.

"Everyone wants Bernie Sanders to be well. Politically, it's good for Warren and Sanders to have each other in the race,” said Adam Green, co-founder of the Progressive Change Campaign Committee, which has endorsed Warren. “One of the biggest things preventing corporate CEOs from dumping tens of millions of dollars in TV ads against Warren or Sanders is that if you knock one out, you help the other. And if you run ads hitting both of them, you just elevate them as the two most clear and transformational progressives."