The day after doctors announced he had aggressive brain cancer, Sen. John McCain said he planned to be back to work soon.

"I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!" the Arizona Republican wrote on Twitter.

One of the many people calling McCain with their support: Former president George W. Bush. Bush said he called the senator this morning to wish him well and to encourage him to fight.

"Instead, he encouraged me," Bush said in a statement. "I was impressed by his spirit and determination. He has devoted his life to this country. Thankfully, he is committed to continuing that service."

McCain has been diagnosed with glioblastoma, a type of brain tumor that is difficult to treat, experts say.

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