Sen. John McCain on Twitter Thursday announced he will be "back soon" in Congress, after his brain tumor diagnosis shook Washington, D.C., Wednesday night.

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

The announcement from the Arizona Republican's office, family and the Mayo Clinic Wednesday that he has glioblastoma, an aggressive form of brain cancer, devastated many of his colleagues, including members of the opposite party. Sen. Patrick Leahy (D-Vermont) said he was "in tears" when he heard the news.

McCain was home in Arizona for surgery on a blood clot above his eye when doctors discovered the tumor. The next step in treatment is likely chemotherapy and radiation, the senator's primary brain doctor said.

McCain's diagnosis comes as the Senate is fighting over the future of health care legislation, with President Trump pushing the Senate to pass a bill before leaving for August recess. Senate Majority Leader Mitch McConnell on Wednesday said the Senate will vote on a motion to proceed with health care legislation next week. Sen. Lindsey Graham (R-South Carolina), one of McCain's good friends in the Senate, was uncertain whether McCain would return before August recess.

"They're going to spend the next few days figuring out medical options," Graham said Thursday morning. "I talked to John twice yesterday, the truth of the matter is no one saw this diagnosis coming."

"If he's physically possible he'll be back as soon as possible," Graham added. "He's got to make a decision here about quality. Showing up is important but I think we all want him here where he can deliver a message. And he wants to come back in the worst way."

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Many in Washington describe McCain as a Senate institution, and one of the body's most powerful members. Sen. Jeff Flake (R-Arizona), noted that McCain was in the Senate when he arrived in the city as an intern.

"I cam here to the Senate in 1987 as an intern, and he was here then," Flake said. "I've never known the Senate without John."