WASHINGTON (CNN) -- Defense Secretary Leon Panetta turned up the heat on Congress Monday, warning that looming automatic budget cuts would undermine national security and set off a financial chain reaction from the hallways of the Pentagon, to the battlefields of Afghanistan, to civilian assembly lines.
The Pentagon already is digesting $450 billion of reductions over the next decade but now fears an additional $600 billion or more in cuts may be imminent if Congress cannot reach a deal on spending.
"The impacts of these cuts would be devastating for the department," Panetta said in a letter to Sens. John McCain, R-Arizona, and Lindsey Graham, R-South Carolina. He said that congressional failure to reach a budget agreement and the resulting so-called sequestration would trigger 23% across-the-board reductions and a halt to many new projects.
"Such a large cut, applied in this indiscriminate manner, would render most of our ship and construction projects unexecutable -- you cannot buy three quarters of a ship or a building -- and seriously damage other modernization efforts," Panetta wrote to the senators.
"We would also be forced to separate many of our civilian personnel involuntarily and, because the reduction would be imposed so quickly, we would almost certainly have to furlough civilians in order to meet the target. These changes would break faith with those who maintain our military and seriously damage readiness."
The cuts would eventually hit combat troops, Panetta said.
"While wartime funding in the Overseas Contingency Operations accounts is not directly affected by the sequester, war efforts would be adversely affected by the severe disruption in the base budgets," Panetta warned. "Contracting personnel would be cut, resulting in delays in the contracts and the contract oversight that support the war. Payroll personnel would be cut, resulting in late payments to wartime vendors, and legal and policy support would be disrupted."
The two senators had written Panetta 10 days ago, asking for details of the potential impact of the automatic cuts on the Defense Department.
"The consequence of a sequester on the Defense Department would set off a swift decline of the United States as the world's leading military power. We are staunchly opposed to this draconian action," the senators said in a joint statement Monday afternoon when they released Panetta's letter. "This is not an outcome that we can live with, and it is certainly not one that we should impose on ourselves. The sequester is a threat to the national security interests of the United States, and it should not be allowed to occur."
Whether the Panetta letter and the fresh warnings from Senators will increase pressure for a budget compromise or step up calls to exempt the Pentagon from cuts remains to be seen.
Panetta has been increasingly outspoken about the possible cuts, although he came to the top Pentagon job with years of budget expertise himself in Congress and the White House and knowing that he was facing tough choices.
At a news conference last week, the secretary of defense painted a bleak picture of what could lie ahead -- a military with a shell but no core. "It's a ship without sailors. It's a brigade without bullets. It's an air wing without enough trained pilots. It's a paper tiger, an Army of barracks, buildings and bombs without enough trained soldiers able to accomplish the mission," Panetta said in his opening remarks at the Pentagon. "It's a force that suffers low morale, poor readiness and is unable to keep up with potential adversaries. In effect, it invites aggression.."
In an addendum to his letters to McCain and Graham, Panetta spelled out new specifics of how reductions "generate significant operational risks: delay response time to crises, conflicts, and disasters; severely limits our ability to be forward deployed and engaged around the world; and assumes unacceptable risk in future combat operations."
And Panetta said that some of the biggest defense projects could face the ax, including those already being tested and some just in early stages of planning. That list included the F35 Joint Strike Fighter, a planned new bomber, the next-generation ballistic submarine, the new littoral combat ship and the new ground combat vehicle the Army and Marines need to replace the humvee.
Halting further development and testing the F35 could generate some $80 billion in savings over 10 years but its supporters say it is a vital next step to upgrade and meet potential threats from China and other rivals.