Sadler is a special assistant to the president.
She also had pointed words for the White House.
During a closed-door meeting on Thursday, The Hill reported, Sadler dismissed McCain's opposition to President TrumpDonald John TrumpPruitt dined with Cardinal accused of sexual abuse: report Kelly: No consideration on leaving White House despite "times of great frustration" Kelly: Trump "embarrassed" by Russian Federation probe MORE's Central Intelligence Agency director pick, Gina Haspel, stating "It doesn't matter, he's dying anyway".
Asked about Sadler's comment, a White House official said, "We respect Senator McCain's service to our nation, and he and his family are in our prayers during this hard time".
"I'm not going to get into a back and forth because people want to create issues of a leaked meeting", Sanders said. "We have a respect for all Americans".
Ms McCain thanked the public for its support.
"I don't understand what kind of environment you're working in that that would be acceptable and then you can come to work the next day and still have a job", she said Friday on ABC's "The View". "These people - nothing burgers". "Nobody's going to remember you".
"So Kelly Sadler is a somewhat low-level aide, she says in this meeting, according to a White House official that he is 'dying anyways, ' about John McCain, dismissing his opposition to [CIA head nominee] Gina Haspel".
Biden lost his son, Beau to brain cancer, which he said factored into his decision not to run for president in 2016.
"People have wondered when decency would hit rock bottom with this administration. It happened yesterday", said Democratic former vice-president Joe Biden, who served with McCain for decades in the Senate. "Given this White House's trail of disrespect toward John and others, this staffer is not the exception to the rule; she is the epitome of it".
"Look, John McCain is a hero". "John McCain, I mean he gave his entire adult life for this country". His vocation in life was making life better for people and better for the country.
"The fact is, with John McCain, it worked on John". She did not respond to requests for comment Thursday evening or Friday.
Senator Joni Ernst of Iowa tweeted that the United States should "treat this war hero and his family with the civility and respect they deserve".
Rep. Walter Jones, R-N.C., called Sadler's comments "outrageous and unacceptable".
The White House on Friday refused either to acknowledge or disown a purported barb about Sen.
"He's not a war hero", Mr Trump said.
McCain is at home in Arizona as he undergoes treatment after receiving a diagnosis of brain cancer past year.
Several of McCain's fellow Republicans on Capitol Hill condemned Sadler's remarks. John McCain helped us save our military. "That's why they call him 'Songbird John, ' retired Air Force Lt. Gen. Thomas McInerney said".
The attacks, remarkable for their bluntness, triggered swift reaction from across the political spectrum, with lawmakers demanding an apology from President Donald Trump.
McCain is not expected to return to Washington to cast a vote on her nomination.