"Cancer doesn't know what it's up against."
News of Arizona Senator John McCain's brain cancer diagnosis shocked Washington Wednesday, prompting well wishes and kind words from politicians from both sides of the party line.
But one of the most moving messages came from the man who defeated McCain in the 2008 presidential election, Barack Obama.
"John McCain is an American hero & one of the bravest fighters I've ever known," Obama tweeted Wednesday. "Cancer doesn't know what it's up against. Give it hell, John."
The tasteful message comes as no surprise — the two men, who faced off in the presidential race nine years ago, have been said to have a "mutual respect" for one another, as seen in this photo from former White House photographer Pete Souza.
The brain tumor, or glioblastoma, was found on Friday while McCain was undergoing a blood clot removal over his left eye, according to medical officials at the Phoenix Mayo Clinic Hospital.
The aggressive cancer is known to spread to other parts of the brain, but McCain is reportedly healing from the surgery "amazingly well," and can start radiation once his incision heals, his office said in a statement.
The senator has had melanomas removed before, in 1993, 2000 and 2002. His daughter, Meghan McCain, tweeted that the news of her father's illness has affected the McCain family deeply.
"It won't surprise you to learn that in all this, the one of us who is most confident and calm is my father."