He added that "unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!"
I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!— John McCain (@SenJohnMcCain) July 20, 2017
Everyone from Pres. Donald Trump and Pres. Barack Obama to the Arizona Cardinals and wide receiver Larry Fitzgerald have sent well wishes to the senior Republican senator from Arizona.
Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. Get well soon. https://t.co/fONWVlmYyz— Donald J. Trump (@realDonaldTrump) July 20, 2017
One theme was consistent across many of the tweets -- that McCain is a tough and worthy adversary in the battle ahead.
Obama told McCain "to give it hell."
John McCain is an American hero & one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John.— Barack Obama (@BarackObama) July 20, 2017
The Cardinals released a statement saying "There is not a person who has embodied toughness and courage throughout his life like John McCain."
Fitzgerald said "they don't come any tougher.
Thinking of you tonight @SenJohnMcCain. They don't come any tougher & no one is better equipped for a fight like this than you.— Larry Fitzgerald (@LarryFitzgerald) July 20, 2017
Arizona Diamondbacks President and CEO Derrick Hall said there was no bigger fan of the team and McCain would "win another battle."
The Phoenix Suns said the team was sending its best to "one of the toughest there is."
The Arizona Coyotes said he was a hero and part of the family.
McCain's counterpart, Sen. Jeff Flake, tweeted that he spoke to McCain on Wednesday night. "Tough diagnosis, even tougher man."
Just spoke to @SenJohnMcCain. Tough diagnosis, but even tougher man.— Jeff Flake (@JeffFlake) July 20, 2017
Arizona's Gov. Doug Ducey said John McCain was "undoubtedly the toughest man in the US Senate" and has served the country "like few ever will."
He has set an example for all Americans in the toughest of fights, in difficult circumstances. I have no doubt he'll do it again. (2/3) https://t.co/rlx3z3dfPl— Doug Ducey (@dougducey) July 20, 2017
His daughter, Megan McCain, shared an emotional note about her father Wednesday, saying that "he is the toughest person I know."