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Sen. John McCain had a message Thursday for his well-wishers and colleagues: "I'll be back soon."

The Arizona Republican tweeted that he'll return to spar in the Senate soon and gave thanks for the support he's received following the news Wednesday night that he has been diagnosed with brain cancer.

"I greatly appreciate the outpouring of support - unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!" McCain, 80, said.

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

McCain underwent a minimally invasive surgical procedure to remove a blood clot above his left eye last Friday, according to a statement from the senator's office. The blood clot was associated with a tumor, which doctors removed.

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The tumor is an aggressive one known as a glioblastoma, a highly malignant form of cancer that can spread quickly due to its association with a large network of blood vessels in the brain.

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The senator is now resting at his Arizona home and considering treatment options.

In the wake of the diagnosis, there was a flood of support for the Arizona Republican, a former Navy pilot and prisoner of war in Vietnam, that spanned both sides of the aisle. The Republican and Democratic leaders in both houses of Congress wished him well, and ex-presidents Barack Obama and George H.W. Bush were among those expressing their support.

President George W. Bush also issued a statement Thursday wishing him a speedy recovery.

"I called Senator John McCain this morning to wish him well and encourage him in his fight. Instead, he encouraged me," Bus said. "I was impressed by his spirit and determination. Laura and I pray for our friend to fully recover and quickly return to the Senate, where his voice and leadership are needed."

Meghan McCain, the senator's daughter, wrote a heartfelt message about the diagnosis on Instagram Wednesday, calling her father "my hero" and "a warrior."

She also said the news has shocked the family and noted that "we live with the anxiety about what comes next" as she asked supporters for their prayers.

"He is the toughest person I know. The cruelest enemy could not break him. The aggressions of political life could not bend him," she wrote. "So he is meeting this challenge as he has every other. Cancer may afflict him in many ways: but it will not make him surrender. Nothing ever has."

McCain, the chairman of the Armed Services Committee, displayed that warrior spirit Thursday when he gave a scathing critique of President Donald Trump in response to a Washington Post report that his administration has decided to end a CIA program to train and arm Syrian rebels fighting president Bashar Al-Assad.

“If these reports are true, the administration is playing right into the hands of Vladimir Putin. Making any concession to Russia, absent a broader strategy for Syria, is irresponsible and short-sighted," McCain said in a statement. "The administration has yet to articulate its vision for Syria beyond the defeat of ISIL, let alone a comprehensive approach to the Middle East."