The Pentagon is bracing for spending cuts far deeper than what it was expecting just a few weeks ago, including the possible elimination of an aircraft carrier group and other weapons programs, as an increasing number of lawmakers float proposals for slashing the once-sacrosanct defense budget.

Defense officials have been warning for months that the Pentagon must prepare for a new era of austerity after a long period of growth that has swelled military spending to its highest level, adjusted for inflation, since World War II.

But as lawmakers and the White House move closer to a grand bargain that could reshape the country’s fiscal priorities, Pentagon budget planners are scrambling to keep up. Military officials said they are girding for the possibility that they will have to reduce projected spending by as much as $800 billion over the next 12 years.

That’s twice the worst-case forecast they confronted as recently as April, when President Obama warned his administration that it might have to cut $400 billion from its national-security budgets over the same time frame.

“We’re doing due diligence on that,” Gen. James E. Cartwright, vice chairman of the Joint Chiefs of Staff, told reporters at a Defense Writers Group breakfast last week. “But the reality is you’re most worried about a deeper cut. Is there another $400 billion beyond the first $400 billion?”

That has opened the door to internal discussion on whether the Pentagon will have to revisit several high-profile weapons programs that until recently were considered safe.

The Navy, for instance, is feeling pressure to cancel its next-generation ballistic-missile submarine and to reduce its fleet of 11 aircraft carriers. The Air Force is facing renewed doubts about its futuristic long-range bomber. And the Army is worried that it will have to shrink the size of its active-duty force even further; the number of soldiers is already planned to drop from 569,000 to 520,000 over the next five years.

Officially, the Pentagon says nothing is off the table when it comes to possible cuts. In April, then-Defense Secretary Robert M. Gates ordered a “comprehensive review” of Pentagon spending, saying he wanted to reconsider the department’s strategic priorities instead of just ordering across-the-board spending reductions.

The results of that review, however, are not scheduled to be released until February, which could render the Pentagon’s recommendations moot if Congress and the White House agree on a deal before then.

New Defense Secretary Leon Panetta, who took office July 1 after Gates’s retirement, has said little publicly in reaction to the various proposals to curtail defense spending. But as a longtime budget expert in Congress and the Clinton administration, he is expected to endorse any agreement hammered out between Obama and lawmakers.

In the past, the Pentagon could count on strong support for ever-rising budgets from Republicans and conservative Democrats. But that has changed rapidly; even some GOP leaders are now calling for once-unthinkable reductions in military spending.

On Monday, for example, Sen. Tom Coburn (R-Okla.) released a plan that would shrink the Pentagon’s planned budgets by $1 trillion over the next decade. Other Republicans, while not as ready to cut military spending that deeply, say it’s more important to avoid tax hikes as they look for solutions to the nation’s fiscal ills.

“Who knows where the needle will end up?” said Thomas Donnelly, director of the Center for Defense Studies at the American Enterprise Institute. “Nobody has defense as a high priority. It’s increasingly looking like everybody wants to toss the military overboard.”

As recently as a year ago, the Pentagon had hoped it could escape relatively untouched. In May 2010, Gates ordered the armed services to scrape together $100 billion in savings over five years in what he called an “efficiencies” drive. But rather than give the money back to the Treasury, Gates said the services could spend the money on new programs instead.

In January, Gates tried again to deflect fiscal pressures by ordering the Pentagon to cut $78 billion in projected spending over five years. But that figure was soon dwarfed by Obama’s directive in April to identify $400 billion more in possible cuts over 12 years.

While those may sound like big numbers, some analysts said they represent a reduction in projected spending only and that the Pentagon’s budget would actually continue to grow slightly, about at the rate of inflation.

Even cuts of $1 trillion over 10 years could be absorbed relatively easily, said Gordon Adams, who oversaw national-security budgets for the Clinton White House and is now a professor at American University.

“It’s not like it would be whacking American’s defense budget to the bone,” he said.

While a reduction of $1 trillion would be significant, he said, it would represent a much smaller-percentage decline in defense spending than what the Pentagon encountered at the end of the Cold War, when its budget declined by 36 percent between 1985 and 1998.

Meanwhile, signs that the Pentagon is entering a period of thrift are becoming apparent.

On Thursday, the Defense Business Board, a private-sector panel that advises the secretary of defense on financial matters, will convene its quarterly meeting to discuss a report on “corporate downsizing applications” that might prove relevant for the military.

“This new era will require a different mindset,” Ashton B. Carter, the Defense Department’s chief weapons buyer, said to an audience at the Brookings Institution.

While expensive weapons programs may grab the most public attention, analysts said the biggest — and toughest — target for spending reductions will be personnel costs, which are rising at unsustainable rates. Also soaring are expenses for overhead, back-office functions, military schools, commissaries and other functions that are distant from the battlefield, said Arnold L. Punaro, a member of the Defense Business Board and a retired general in the Marine Corps.

“The reason we have a Department of Defense is to break things and kill people,” Punaro said. “The problem we have now is that so much of the money is not going to the people who are breaking things and killing people.”