The Capitol Hill office of Sen. John McCain, R-Ariz., is seen in Washington, Thursday, July 20, 2017, after news that he has been diagnosed with brain cancer. A member of Congress for more than three decades, McCain was the 2008 Republican presidential nominee and a Vietnam prisoner of war. (AP Photo/J. Scott Applewhite)

The Capitol Hill office of Sen. John McCain, R-Ariz., is seen in Washington, Thursday, July 20, 2017, after news that he has been diagnosed with brain cancer. A member of Congress for more than three decades, McCain was the 2008 Republican presidential nominee and a Vietnam prisoner of war. (AP Photo/J. Scott Applewhite)

WASHINGTON (AP) — Sen. John McCain’s treatment for brain cancer could keep him out of Washington for weeks, perhaps months, and yet it’s unlikely anyone will challenge his extended leave.

Congress has a long tradition in which no one questions ailing lawmakers taking time to recover. For starters, it’s just poor form. And, frankly, it’s up to the stricken member of Congress and their doctors to decide when — or even if — they return to work. Some have recuperated away from the Capitol for a year or more.

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It’s an unwritten courtesy that often doesn’t extend to the real working world where employees are forced to file for medical disability or take unpaid leave.

Julie Tarallo, McCain’s spokeswoman, said Friday that “further consultations with Sen. McCain’s Mayo Clinic care team will indicate when he will return to the United States Senate.”

McCain had taken to Twitter on Thursday promising a quick return.

“Unfortunately for my sparring partners in Congress, I’ll be back soon, so stand-by!” said the six-term Arizona Republican and 2008 GOP presidential nominee.

The 80-year-old McCain was diagnosed with glioblastoma, an aggressive type of brain cancer, according to doctors at the Mayo Clinic in Phoenix, who had removed a blood clot above his left eye last Friday. He and his family are weighing his treatment, including radiation and chemotherapy.

In the immediate aftermath of McCain’s diagnosis, Republicans wouldn’t speculate about what the temporary loss of McCain’s vote would mean. But McCain’s absence complicates Majority Leader Mitch McConnell’s plans for a Senate vote on a GOP health care bill to erase much of the Affordable Care Act. A vote is possible on Tuesday, but GOP defections plus McCain’s likely absence could sink any chance even to get started.

News of his diagnosis prompted an outpouring of support and prayers from Democrats and Republicans. No one suggested he step down — except former GOP rival Kelli Ward.

In a statement, Ward said, “When the time comes that Senator McCain can no longer perform his duties in the Senate at full capacity, he owes it to the people of Arizona to step aside.”

McCain easily defeated Ward in last year’s primary.

McCain wouldn’t be the first lawmaker this year to miss votes, hearings and other legislative action. Republican Sen. Johnny Isakson remained in Georgia for several weeks earlier this year as he underwent two back surgeries and recuperated. Isakson missed the vote on confirming Supreme Court Justice Neil Gorsuch.

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In January 2012, then-Sen. Mark Kirk, R-Ill. suffered a major stroke and didn’t return for almost a full year, making a dramatic entrance by climbing the steps of the Capitol on the opening day of the following Congress.

In a lawmaker’s absence, congressional staff keep the office operating, send out news releases — one from McCain on Thursday blasted the Trump administration’s Syria policy — and respond to constituents.

Absences can leave the margin of control on a razor’s edge.

The month after Democrats won back the Senate in 2006, South Dakota Democrat Tim Johnson had a near-fatal episode of bleeding in his brain that, at the time, threatened to shift the Senate’s margin from 51-49 Democratic to 50-50 GOP control with Republican Vice President Dick Cheney the deciding vote. Johnson recovered but was away from the Senate for almost nine months.

McCain is battling the same form of cancer that claimed the life of Sen. Ted Kennedy, D-Mass., in August 2009. Kennedy was away from the Senate for extended stretches but returned on occasion to vote.

“There were times when Senator Reid had to juggle things because he had two senators absent, Senator Kennedy and Senator Byrd,” said longtime former Senate aide Jim Manley, who worked for both Kennedy and then-Majority Leader Harry Reid, D-Nev. “Having said that, it really never, with a handful of exceptions, proved to be that big of a problem.”

Kennedy also delegated some of his responsibilities as chairman of the Health, Education, Labor, and Pensions Committee by farming out responsibility for bills before the panel to colleagues such as then-Sens. Chris Dodd, D-Conn., and Barbara Mikulski, D-Md. McCain has had Sen. Jim Inhofe, R-Okla., handle his duties as Armed Services Committee chairman.

Unclear is whether Inhofe will steer the sweeping defense policy bill if the Senate begins debate in August.

And, if legislative necessity should dictate that McCain return for a crucial, dramatic vote, there’s precedent. Kennedy, who mostly stayed away from the chamber for fear of infection, returned to the Senate in July 2008 for a key vote. During McCain’s first term, Sen. Pete Wilson, R-Calif., recovering from an emergency appendectomy, was wheeled in on a stretcher to cast the deciding vote on a GOP budget plan.

And in 1964, California Democrat Clair Engle, whose own bout with brain cancer rendered him unable to speak, was wheeled into the Senate to vote for the landmark Civil Rights Act. Engle pointed to his eye and tried to mouth “aye,” according to newspaper accounts at the time.

Some senators were away from the chamber for years. Karl Mundt, R-S.D., suffered a stroke in late 1969 and refused to resign and allow a GOP replacement to be named. He held the seat until January 1973 and was replaced by Democrat Jim Abourezk.

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Associated Press writer Michael Blood in Los Angeles contributed to this report.