Press reports this week revealed that U.S. Senator John McCain is battling brain cancer. Shortly after the news broke, former President Barack Obama tweeted out a message of support to the 80-year-old Republican, who ran against him in the 2008 election.

Though the two were often on opposing sides, Obama praised the ailing Vietnam War veteran as "an American hero and one of the bravest fighters I've ever known."

"Cancer doesn't know what it's up against," the president added. "Give it hell, John."

Several hours later Donald Trump tweeted out his own statement, encouraging McCain to "get well soon."

McCain was diagnosed with glioblastoma, the most aggressive form of brain cancer there is. The survival rate is about 12 to 15 months, with fewer than 5% of patients surviving longer than five years. But, hey, "Get well soon!"

In 2015 Trump declared McCain was not a war hero, saying "He was a war hero just because he was captured. I like people who weren’t captured.”

Even Sarah Palin mustered a warm message for her former running mate, telling Tucker Carlson, "If I were going to war, I wouldn’t just want him on my side, I’d want him leading the charge."

Though McCain had gone in for a relatively minimal procedure to remove a blood clot from behind his left eye, doctors at the Mayo Clinic in Phoenix confirmed the presence of a very aggressive tumor. His absence led Senate Majority Leader Mitch McConnell to delay a vote on the Republicans' replacement plan for the Affordable Care Act.

Home recuperating from the procedure, McCain is expected to begin radiation in a few weeks.

“Senator McCain appreciates the outpouring of support he has received over the last few days," his office said in a statement. "He is in good spirits as he continues to recover at home with his family in Arizona. He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective."