WASHINGTON — Weeks after agreeing to impose mandatory spending cuts on the federal government in exchange for increasing the debt ceiling, lawmakers are hard at work to overturn a key element of the deal — the threat of automatic, steep cuts in the defense budget.

The possibility of defense cuts — what budget insiders call a trigger mechanism — was intended to spur Republicans and Democrats to agree on a plan to reduce the deficit by $1.5 trillion over 10 years. Instead, Congress increasingly seems likely to scuttle the cuts even without a deficit deal.

"It feeds into the notion that everyone is having, but not saying, which is that the trigger is a complete phony thing," said Jim Kessler, a vice president at Third Way, a moderate Democratic think tank. "Congress has built up a reputation for avoiding any real decision."

To be sure, the Joint Committee on Deficit Reduction, co-chaired by Sen. Patty Murray, D-Wash., and Rep. Jeb Hensarling, R-Texas, has been working behind closed doors for the past two months trying to find common ground.

And the effort to undo the automatic cuts is at odds with the public stance of congressional leaders, including House Speaker John Boehner, R-Ohio. The speaker orchestrated the summer debt accord with President Obama and has pressed the so-called supercommittee to reach as big a deficit deal as possible.

The 12-member panel — six Republicans and six Democrats — will hold a public hearing Wednesday as it works toward its Nov. 23 deadline to produce a bipartisan package. Indications are that the committee tentatively has identified preliminary cuts but is still short of its $1.5 trillion goal.

If the committee comes up with less than $1.2 trillion in proposed cuts over 10 years, the shortfall is supposed to be made up by automatic cuts that would start in 2013, split evenly between defense and nondefense spending.

But public statements and private comments from key defense champions and their allies, backed by the intense lobbying of the defense industry, have thrown into doubt whether those reductions will come to pass.

Defense hawks have been unbridled in their mission. Arizona Sen. Jon Kyl, the Senate's No. 2 Republican, said he would walk away from his position on the supercommittee if defense cuts were part of its recommendation.

Sen. John McCain, R-Ariz., who is not on the committee, said he would try to roll back the mandatory defense cuts before they could kick in.

Rep. Howard "Buck" McKeon, R-Calif., chairman of the House Armed Services Committee, suggested steep mandatory cuts could require the military to institute the draft. If forced to choose, McKeon said, he would prefer new taxes — something he has never voted for.

A Virginia Republican is circulating a resolution of opposition with dozens of co-sponsors.

Even though some tea-party lawmakers say all federal agencies must be trimmed, the defense hawks are receiving a boost from top Pentagon brass, including Defense Secretary Leon Panetta, who warned that making the required reductions would be "shooting ourselves in the head."

"It would be the dumbest thing," Sen. Lindsey Graham, R-S.C., said during an appearance by Panetta at a Senate hearing last month. "I am disappointed in my Republican Party for allowing that to be part of the puzzle."

The White House acknowledges the trigger mechanism is "not ideal policy," according to Meg Reilly, a spokeswoman for the Office of Management and Budget. But the administration says its purpose was to be an unpopular possibility that would motivate lawmakers to reach an agreement.

"It's meant to provide a powerful incentive for Congress to do its job and pass balanced, responsible deficit reduction," Reilly said. "Congress has a responsibility to make cuts to defense prudently to ensure that our national-security efforts are not compromised."

Supercommittee Democrats have concerns that the panel's work could be undermined by suggestions that defense is off the table, according to an aide familiar with the deliberations. With Republicans already refusing new taxes, that would force disproportionate cuts on health, education and federal programs for the poor.

Because the mandatory reductions would not go into effect until 2013, Congress has a full year to wrestle with the issue. The presidential election likely will influence that debate, and Congress still will have a few months to act afterward — allowing members to put off a formal decision on the triggers until after the elections. Many doubt the triggers ever will be pulled — or that a move to protect only defense would succeed.

"The bullets don't even hit until a year later — you've got a year to correct it," said Sen. Carl Levin, D-Mich., chairman of the Armed Services Committee, who doubts the political climate would allow defense to be spared at the expense of domestic accounts. "If you want to start over again, that's fair. But I don't think you'd be able to get votes to just undo part of it."

Leaders of both parties are concerned that, if the supercommittee fails, financial markets will be thrown into turmoil over Congress' inability to tackle big problems. The nation's once-stellar credit rating — which one credit agency downgraded this summer — could be eroded further. That could lead to higher interest rates for ordinary Americans on virtually every aspect of consumer lending, further imperiling the sluggish economy.

McCain, a top party leader on defense issues, downplayed his influence on the supercommittee. But if the panel fails, the Arizona senator is optimistic his backup plan to protect the defense budget would find broad bipartisan support.

"We think we could succeed," McCain said.