Washington (CNN) The news of Sen. John McCain's brain cancer diagnosis was immediately met with shock and grief Wednesday night, with many taking to social media to express support for the Arizona Republican.

Several Republican senators learned of the news during a meeting about health care, North Dakota Sen. John Hoeven told CNN, adding that there was "stunned disbelief" in the room and they prayed for McCain.

"Senator John McCain has always been a fighter," President Donald Trump said in a statement. "Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. Get well soon."

A White House official said Trump called McCain following word Wednesday night of the senator's diagnosis.

Vice President Mike Pence said "Cancer picked on the wrong guy" in a tweet Wednesday evening, adding that he and his wife, Karen, are praying for him. "McCain is a fighter & he'll win this fight too. God bless!"

Former President Barack Obama, McCain's 2008 presidential campaign opponent, tweeted his support

"Cancer doesn't know what it's up against. Give it hell, John," the 44th president wrote.

Other ex-presidents offered warm thoughts as well.

Bill Clinton wrote on Twitter , "As he's shown his entire life, don't bet against John McCain. Best wishes to him for a swift recovery."

George W. Bush spoke with McCain Thursday. "I called Senator John McCain this morning to wish him well and encourage him in his fight. Instead, he encouraged me. I was impressed by his spirit and determination. He has devoted his life to his country. Thankfully, he is committed to continuing that service," he said in a statement.

Bush's father, George H.W. Bush, also shared support : "The Hanoi Hilton couldn't break John McCain's spirit many years ago, so Barbara and I know -- with confidence -- he and his family will meet this latest battle in his singular life of service with courage and determination," he said in a statement.

Former Vice President Joe Biden, who has been friends with McCain for 40 years, said on Twitter , "He's gotten through so much difficulty with so much grace. He is strong - and he will beat this."

Senate Majority Leader Mitch McConnell called McCain "a hero to our conference and a hero to our country."

"He has never shied from a fight and I know that he will face this challenge with the same extraordinary courage that has characterized his life," McConnell said in a statement. "We all look forward to seeing this American hero again soon."

Louisiana Rep. Steve Scalise, who is recovering from surgeries to treat the gunshot wound he recently sustained at a congressional baseball practice, said on Twitter he is "Praying for my friend @SenJohnMcCain, one of the toughest people I know."

Sen. Lindsey Graham, a South Carolina Republican who is one of McCain's closest friends on the Hill, said the Arizona senator told him he "wants to get back and get to work."

"This disease has never had a more worthy opponent," Graham said.

House Speaker Paul Ryan called McCain a "warrior."

"I know John is going to fight this with the same sheer force of will that has earned him the admiration of the nation. And all of us, not as Republicans or Democrats, but as Americans, are behind him," he said in a statement

House Minority Leader Nancy Pelosi said she is "privileged to call him a friend."

".@SenJohnMcCain is a hero, a patriot and a fighter. I am privileged to call him a friend. Paul & I pray for his recovery and his family."

Arizona's other senator, Jeff Flake, wrote in a tweet , "Just spoke to @SenJohnMcCain. Tough diagnosis, but even tougher man."

Just spoke to @SenJohnMcCain. Tough diagnosis, but even tougher man. — Jeff Flake (@JeffFlake) July 20, 2017

McCain's 2008 running mate, former Alaska Gov. Sarah Palin, shared a photo to Facebook of the two at a campaign event, adding "he'll face this diagnosis with courage and strength."

Arizona Gov. Doug Ducey echoed others in calling McCain, who spent more than five years as a prisoner of war in Vietnam, "undoubtedly the toughest man in the US Senate."

"He is an American hero and has served our country like few ever will," Ducey said on Twitter . "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."

Hillary Clinton expressed her support for the McCain family.

"John McCain is as tough as they come," she said in a tweet . "Thinking of John, Cindy, their wonderful children, & their whole family tonight."

McCain's daughter, Meghan McCain, responded to the news in a statement.

"The news of my father's illness has affected every one of us in the McCain family. My grandmother, mother, brothers, sisters and I have all endured the shock of the news, and now we live with the anxiety about what comes next," she said. "It won't surprise you to learn that in all this, the one of us who is most confident and calm is my father. He is the toughest person I know."