Trump disclosed that he and first lady Melania Trump tested positive for Covid-19 in a tweet early Friday morning; the president is now hospitalized and has required supplemental oxygen. It is not known where the president contracted the virus, but he and Biden both attended the first presidential debate in Cleveland last week.

Biden has returned to the campaign trail, but has suspended all negative advertising since the president tested positive for Covid-19. Sanders said Sunday no one from the White House has reached out to the Biden campaign to tell them that they had potentially been exposed at Tuesday's debate.

"No, we haven't heard, to my knowledge, from the Trump campaign or the White House," she sad. "But the reality is that Vice President Biden was not exposed. According to CDC guidance, less than six feet away is dangerous. Vice President Biden was always more than six feet away from President Trump."

Sanders also noted that Joe Biden, Jill Biden, Sen. Kamala Harris and her husband, Doug Emhoff, all traveled on Friday, "but not one person got on a plane until we were sure that everyone was negative."

CNN host Jake Tapper pressed Sanders on why Joe Biden had not been tested every day, cautioning that the six-feet apart social distancing guideline is not a guarantee against infection.

"What does regularly mean? When you say Mrs. Biden and Vice President Biden is tested regularly, what does that mean? It's not like a specific number. That could be every week, it could be every month. Why not every day?" he said.

"He is tested before we travel, Jake," Sanders responded.

Sanders also said that Biden plans to be at the next debate on Oct. 15 in Miami. "We hope that [Trump] is medically able to participate, that is up to his doctors to clear him," she said. "But Joe Biden will be at that debate."

Trump senior adviser Jason Miller appeared on NBC's "Meet the Press" on Sunday, where he said the Trump campaign is pressing forward and does not want to hold remote debates.

"No, we're in a campaign. We have a month to go. We see Joe Biden and Kamala Harris out there campaigning," Miller said. "Certainly, they're not asking for remote debate."

Vice President Mike Pence, Miller said, will hit the campaign trail in Arizona and likely Nevada, while the president recovers from the coronavirus.

"He's going to have a very full, aggressive schedule," he added.