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The chairman of the House Armed Services Committee on Friday issued another warning to members of the bipartisan debt supercommittee urging them to come to an agreement in order to avoid a “devastating” across-the-board cut to Pentagon spending in January 2013.

“Hope is not a strategy that our senior military leadership can rely on,” Rep. Buck McKeon (R-Calif.) wrote in a letter to the co-chairs of the 12-member debt panel, Rep. Jeb Hensarling (R-Texas) and Sen. Patty Murray (D-Wash.). “They are obligated to plan for the worst-case scenario. They will not wait until December 2012 in hopes that things get better.”

He added: “Unless we act today, the dismantling of the greatest Armed Forces in history could begin tomorrow.”

McKeon, other members of Congress and Pentagon officials have argued since the supercommittee’s inception that the consequences of a failure by the panel would hit defense spending particularly hard if the across-the-board cut were to go into effect in 2013.

But Friday’s letter marks a shift in that message, as it underscores that even if members of Congress seek to undo the across-the-board cut, officials must make long-term spending decisions based on the information that is available at the time.

“As the FY13 budget is due early next year, a failed agreement would force the Defense Department to begin deep and irrevocable cuts to our Armed Forces next year in order to reap the savings in 2013,” McKeon wrote. “Once the process is set in motion, it will be difficult — if not impossible — to stop.”

Democrats on the bipartisan panel late Friday afternoon rejected an offer from Republicans that would have fallen short of the committee’s mandate to achieve $1.2 trillion in deficit savings over the next 10 years.

That means that five days out from its Nov. 23 deadline, the joint committee remains deadlocked, and negotiations among the 12 lawmakers look to continue throughout the weekend.

Some members of both parties have downplayed the impact a failure by the panel would have, noting that $1.2 trillion in deficit savings will be enacted anyway due to the “trigger” called for in the August debt deal and that members in theory would have 13 months until January 2013 to try to come to an agreement on an alternative way to distribute those cuts.

But McKeon’s letter Friday warns against that line of thinking, arguing that the impact on national security would be “immediate.”