Sen. John McCain (R-Ariz.), 81, at the Capitol earlier this month. (Bill O’Leary/The Washington Post)

In November, Sen. Orrin G. Hatch, who is 83, was at the helm when the Senate’s massive tax bill came through the Finance Committee. But Hatch also deputized four younger Republicans on the panel to serve as de facto co-chairmen over various parts of the legislation.

This week, with a compromise bill marching toward final passage in both chambers, the House has to vote first — because a pair of senators, Thad Cochran (R-Miss.) and John McCain (R-Ariz.), are recuperating from, respectively, non-melanoma skin surgery and the side effects of cancer treatments.

Hatch’s advisers say his move demonstrates a keen sense of coalition building, and aides and friends to Cochran, 80, and McCain, 81, contend that their bosses should be back in the Senate before long.

But here’s something else to consider: All three are exemplars of an institution that has become, by one measure, the oldest Senate ever. Eight octogenarians currently serve, nearly twice as many as ever before, according to records maintained by the Senate Historical Office. Another handful of senators are at least 75.

For decades, older members of Congress have brushed aside questions about their fitness for office. They have defended their health and faculties, and some have implied that those who inquire are ageists who don’t understand that America is growing older.

Sen. Patrick J. Leahy D-Vt.), 77, late last month at his office at the Capitol, where he has served since 1975. (Mari Matsuri/AFP/Getty Images)

But the change in schedule for the tax bill is at least the third time this year that Senate leaders paused action to accommodate ailing colleagues. It is now clear that the large number of older senators in positions of power is taking a toll on the operations of Washington.

First came the unsuccessful repeal effort of the Affordable Care Act, delayed until McCain returned to Washington following his initial diagnosis and surgery. McCain as of Saturday was at Walter Reed National Military Medical Center, suffering from the side effects of an aggressive round of chemotherapy and radiation.

In mid-October, Senate Majority Leader Mitch McConnell (R-Ky.) waited out Cochran’s return from a debilitating urinary tract infection to pass a budget that was essential to setting up the framework for passage of the tax plan.

To be fair, some of these seniors are healthier and wittier than their junior colleagues — Sen. Charles E. Grassley, 84, runs four times a week — and some exemplify an aging society where professionals function at high levels well beyond traditional retirement age.

But, collectively, the institution is struggling amid the weight of so many seniors holding such critical positions. Some colleagues say that it has become too hard for senators to walk away at the right time.

“What happens around here, it’s pretty seductive,” said Sen. Bob Corker (R-Tenn.), a 65-year-old who decided this fall he would retire at the end of 2018. “The longer you’re here, the more influence that you have. So it causes you to want to stay and stay and stay.”

All eight of today’s 80-something senators hold top posts. Cochran, first elected in 1978, controls the purse strings of every federal agency as chairman of the Appropriations Committee. McCain, who oversees the Pentagon on the Armed Services Committee, temporarily handed over the gavel this week to Sen. James M. Inhofe (R-Okla.), who ran hearings on Middle East security issues. Inhofe, 83, was until early this year the chairman of the Environment Committee.

Some senators get angry when asked about their age. “If I can run three miles four times a week, I’ll be running for reelection,” said Grassley (R-Iowa), pondering a bid for reelection in 2022, when he will be 89. “I ran just two miles this morning, but don’t read too much into that. Because it’s the same day I packed my suitcase to go home, and I needed a little extra time.”

Informed that this was the oldest Senate ever, Sen. Patrick J. Leahy (D-Vt.) chuckled. “I feel younger every day,” the 77-year-old joked.

Leahy, first elected in 1974, won reelection last year to a term that will end when he is 82. He has a ritual, around every birthday, to test his physical fitness. He goes scuba diving, first swimming down to the depth of his new age, doing a somersault underwater to the 90-foot mark, then swimming to the surface.

“If I reach the point that I can’t go scuba diving and do my somersaults, that will be one clear indication,” Leahy said of knowing when to retire.

For the Senate’s first 100 years, no one ever served into their 80s, according to Senate Historical Office data. Over the next 100 years, there were a few brief moments with two or three. Only 49 senators have ever turned 80 in office, and 15 of those came in the past 20 years.

It’s a very sensitive topic. When Leahy noted that “some in both parties stayed too long,” his chief of staff interjected to say the senator was not referring to any of today’s senators.

Some senators face pressure to stay in office from former staffers whose K Street livelihood is in large part connected to clients with interests before that senator. Sometimes it comes from leaders back home who want their states to reap the benefits of Senate seniority.

Trent Lott, the Republican former Senate majority leader, said his friends in Mississippi are worried about Cochran’s health, but he tells them that the Appropriations chairmanship is too valuable to pass up: “Some people say, ‘Is it time for Thad to come home?’ I say, ‘No! We should keep that chairmanship until the bloody last day that we can.’ ”

Now running the lobbying shop Squire Patton Boggs, Lott gives a presentation to senators who decide it is time to retire. “It’s called the wheel of fortune. And every spoke represents what you can do when you get a real life,” he said, with options ranging from joining a lobbying shop to teaching.

“It’s hard to know when it’s time to move on,” Lott added.

President Trump, 71, is pressing Hatch (R-Utah) to ask the voters in his state to elect him to an eighth six-year term that would end when he is 90 and bring his service in the Senate to 48 years. Sen. Dianne Feinstein (D-Calif.), the oldest senator, is already running for reelection to a fifth full term that would last until she is 91.

Cochran, when he has appeared in the Senate in recent months, has been attended by a staffer who sits next to him and alerts the senator when it is time to vote. McCain, who appeared in good health when he won a sixth term last year, in the past few weeks has relied on staff to push him in a wheelchair to Senate votes.

Corker says it’s easy to grow comfortable with the trappings of power and a large staff that, as senators grow older, sometimes functions like a team of aides in a retirement home.

“Think about it,” he said. “If you’re older, you’ve got more influence, and you’ve got an entire staff of people that, in essence, take care of you every day. Where else, where else can you obtain that? Seriously?”

Yet for him, the decision to retire was clear. He wanted to leave on his own terms, at an age, 65, when he could start another chapter of his life. There are fewer senators than ever making that choice.

“It is a lot of work, if you really want to do your job right,” Leahy said. “You’ve got to be able to do that. If you can’t, it’s not fair to your state or the U.S. Senate to stay here.”

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Correction: An earlier version of this report misstated Inhofe’s current role on the Senate Environment Commission. He is a member but no longer chairman.