Sen. John McCain was his usual irascible self, frustrated with the nonanswers he was getting from the witness, whom he was increasingly treating as hostile.

Who, the chairman of the Armed Services Committee demanded, had been fired over the failure of the Future Combat System, a program that had gone so far off the rails that it was canceled after billions of dollars had been wasted on it?

“When I go to town hall meetings and tell my constituents that we lose $6 billion and there’s not been anyone fired or replaced or a new way of doing things, they’re not really very happy,” McCain said to Ellen Lord during a December hearing, shortly after she had been installed as the Pentagon’s acquisition’s chief. “So the next time you come before this committee — and you will — I want to know what you’ve done besides say, ‘We don’t know who’s responsible.’ ”

Nobody does oversight of the Pentagon’s budget like McCain, R-Ariz. Nobody holds the brass accountable with the same vigor and, at times, vitriol. But now that President Donald Trump has proposed a massive Pentagon spending plan, a $716 billion whopper that is a 13 percent increase over this year, McCain, 81, is sidelined by a deadly form of brain cancer. Although he is reportedly doing well and engaged with the daily events in Washington and the world, he has remained in Arizona since mid-December. And it’s unclear when he’ll return.

His absence comes as Congress is about to begin the arduous work of carving up the White House’s proposed spending plan, and watchdogs and budget analysts wonder who will stand in for McCain, particularly when it comes to looking after the billions of dollars destined for a salivating contractor industry.

“You really can’t overstate how influential Senator McCain has been on defense policy over the past couple of decades,” said Todd Harrison, a senior fellow at the Center for Strategic and International Studies. “Quite frankly, there is no one who can easily step into his shoes and fill that role.”

Many in Congress take their oversight role seriously. But few turn routine congressional hearings into ritualized grillings of some of Washington’s most powerful, a rite of passage for procurement officials, generals and defense contractors, regardless of which party is in power. In addition to those public lashings, McCain also publishes a report titled “America’s Most Wasted,” which he says is designed to expose and shame “wasteful and outrageous government spending — all paid for by you, the American taxpayer.”

His reports have taken aim at programs big and small. There was the $15,000 program from the Environmental Protection Agency “to study pollution from your backyard BBQ.” The $872,164 “to study how children cross the street.” And the $14 million for a “duplicative catfish inspection office.”

But nothing gets McCain as fired up as when a big Pentagon item goes over budget, as they so often do.

He has called the F-35 Joint Strike Fighter program “both a scandal and a tragedy” and “the poster child for acquisition malpractice.” He said the Marine One presidential helicopter program “was an example of the procurement process gone amok.” He blasted the Navy’s Gerald R. Ford aircraft carrier program as “one of the most spectacular acquisition debacles in recent memory.”

Before his inauguration, Trump, too, took aim at both the Air Force One and F-35 programs, threatening to cancel one and squeeze the other. But since his tweets sent shock waves through the defense industry, the president hasn’t gone after particular programs. McCain, however, has remained dogged.

McCain did not respond to a request for comment. While he is treated in Arizona, his staff continues to stay busy, sending out news releases at a furious clip. McCain, friends and colleagues have said, remains active, tweeting recently about his concerns over the Ford-class aircraft carrier and praising the budget deal. On Tuesday, he wished a “Happy #ValentinesDay” to his wife.

Sen. James M. Inhofe of Oklahoma, the No. 2 Republican on the Armed Services Committee said that even though he has been running the meetings, McCain is still very much in charge. “We’re chairing the meetings consistent with what John feels we should be doing,” Inhofe said during a C-Span interview last month. “He’s calling the shots, and I’m showing up.”

On the House side, the Armed Services Committee is run by Rep. Mac Thornberry, R-Texas, considered a thoughtful and diligent chairman, who has led an effort to reform the government procurement process. But he is far more low-key than McCain.

McCain is “watching these hearings and sending in guidance,” said Arnold Punaro, a retired Marine Corps major general, who runs a defense consulting firm. “If anyone can beat the odds, it’s him. . . . You don’t replace a John McCain. He is sui generis, one-of-a-kind unique.”

But his absence on the Armed Services Committee is a concern for watchdog groups, and a relief for those who fear his wrath.

McCain has been known to rip into generals and admirals, whose service and chest full of medals often earn them a pass in Congress. With his long military heritage, his reputation as scion of war heroes and his storied tenure on the Hill, McCain’s ire can have more firepower than the legions of inspector general and Government Accountability Office reports that flood the Capitol.

In a town where it’s often said that power is the most lucrative currency, McCain offers something else: the ability to instill fear.

Since so few members of Congress today have served in the military, they’re unable or unwilling to challenge the institution, said Mark Thompson, a longtime Pentagon reporter now at the Project on Government Oversight. “These folks don’t know the military, and, more importantly, they are cowed by the military,” he said. “They have a glint of guilt, and tend to give the military what it wants without a hard scrubbing. The military knows this, and it’s laughing all the way to the Treasury.”

Sometimes McCain’s ire can turn to fury and he goes overboard, some defense and industry officials said, with critiques that can be unjustly harsh and punitive. “Many times, he gets it right and his criticisms are spot on,” Harrison said. “But sometimes he gets it wrong, and is a bit unfair.”

It’s not just McCain’s absence that is an impediment to congressional oversight, but the fact that this is an election year in Congress, a distraction that “always leads to a stall in governing,” said Mackenzie Eaglen, a defense analyst at the American Enterprise Institute. “Bills tend to stop moving by midsummer. August recess is elongated for campaigning. So difficult questions are always kicked to after the election.”

She summed up the defense industry’s response to the spending plan with one word: “thrilled.”

Trump’s budget is ripe for abuse, said Steve Ellis, a vice president at the advocacy group Taxpayers for Common Sense.

“Just shoveling cash in to the Pentagon is recipe for waste going forward,” he said.

His team of 13 is poised and ready, he said, to be on the watch for wasteful spending. This time of year, budget season, “is our Olympics,” he said.

In McCain’s absence, “other lawmakers are going to have to step up,” he said.