For weeks, Defense Secretary Leon Panetta has been urging the supercommittee to do what it takes to avoid the threat of sequestration and spare the Pentagon from further cuts.

But what if the panel fails?

Sequestration, which Panetta has described as a “doomsday” mechanism, would mean across-the-board cuts beginning in fiscal 2013 under existing statute. But even if it comes to that — and there’s plenty of reason to think it won’t — the Pentagon wants some flexibility in applying the roughly $500 billion in further cuts over the next 10 years.

“If Congress agrees by vote, we would have the flexibility to apply sequester cuts as the administration recommends,” Panetta wrote recently in a fact sheet sent to the chairman of the Senate Armed Services Committee, Sen. Carl Levin (D-Mich.), and the panel”s ranking Republican, Sen. John McCain (R-Ariz.).

“Even with flexibility,” Panetta was quick to add, “cuts of this magnitude would be highly disruptive.”

Republicans have made clear that they will seek to protect the Defense Department from cuts if sequestration fails. Among other efforts, McCain and Sen. Lindsey Graham (S.C.) have drafted a bill that would try to offset some of the reductions by cutting congressional pay and imposing 5 percent across-the-board cuts to other federal spending.

In his fact sheet, Panetta said that for fiscal 2013, when the cuts would take effect, the reductions would delay production and raise the procurement costs for the F-35 Joint Strike Fighter, the Army’s new ground combat vehicle and Navy ships and aircraft. In addition, civilian employees would have to be furloughed for a month or more and training by all services would be curtailed.

While funds that pay for overseas military operations in Afghanistan, Iraq and elsewhere are not affected by sequestration, Panetta said the disruptions caused in the core Pentagon budget by late approval of contracts and perhaps payrolls “would have adverse effect on our ability to support the Afghan war.”