WASHINGTON -- Former U.S. Vice-President Joe Biden sought to console the daughter of ailing Sen. John McCain after she began crying while discussing her father's battle with brain cancer on ABC's "The View."

Meghan McCain, a panelist on the program, she hadn't been able to get through his new memoir, "Promise Me, Dad," which centres on the 2015 death of his son, Beau, from an aggressive tumour called glioblastoma. Doctors diagnosed Arizona Republican Sen. John McCain with the same type of tumour this past summer.

"I think about Beau almost every day and I was told that this doesn't get easier but that you cultivate the tools to work with this and live with this," Meghan McCain said, her voice breaking. "I know you and your family have been through tragedy I couldn't conceive of."

Biden, who served with John McCain in the Senate, stood up and moved from his seat on the set to sit next to her and hold her hand. He told Meghan McCain not to lose hope and that a medical breakthrough is possible.

"And it can happen tomorrow," Biden said, adding that if anyone can beat brain cancer it's John McCain.

McCain, the Armed Services Committee chairman, has missed several votes in the Senate this week and didn't attend a White House ceremony Tuesday that he was invited to. His office didn't respond to an email from The Associated Press.

Now in his sixth Senate term, McCain, 81, underwent surgery in mid-July to remove a 2-inch (51-millimeter) blood clot in his brain after being diagnosed with glioblastoma. He rebounded quickly, however, returning to Washington and entering the Senate on July 25 to a standing ovation from his colleagues.

In a dramatic turn, he cast a deciding vote against the Republican health care bill -- a move that drew the wrath of President Donald Trump and conservatives. McCain's vote scuttled the seven-year effort by the GOP to dismantle much of President Barack Obama's health care law.

But McCain's condition has appeared to worsen in recent weeks. He suffered a minor tear in his right Achilles tendon, forcing him to wear a walking brace. McCain eventually began using a wheelchair with members of his staff pushing him where he needed to go.

As a Navy pilot, McCain lived through a July 1967 fire that killed 134 sailors aboard the aircraft carrier USS Forrestal in the Gulf of Tonkin during the Vietnam War. The following October, his plane was shot down during a bombing mission over Hanoi. He spent more than five years as a prisoner of war. McCain also has survived several bouts with melanoma, a dangerous skin cancer.

Biden, 74, considered a run for the Oval Office in 2016, but decided against it, later citing the trauma of his son death keeping him from the race. During his appearance on "The View," Biden, a former Democratic senator from Delaware, recounted his long friendship with John McCain and how McCain had befriended Beau many years earlier when he served as a Navy liaison officer to the Senate.

Biden also laughed while remembering their political clashes, with the two Senate heavyweights going toe to toe.

"Her dad goes after me hammer and tong," Biden told the audience. But he also said that, even now, if he called John McCain and asked for help, he'd be there for him.

Meghan McCain thanked Biden later, tweeting she had no words to convey her "immense gratitude."

"Your strength, hope and fortitude are an inspiration to me and so many others daily," she wrote.