WASHINGTON — “Unthinkable,” declares Republican presidential nominee Mitt Romney. “A disaster,” predicts Defense Secretary Leon E. Panetta. “Devastating,” agrees Sen. John McCain. “Deeply destructive,” warns President Obama.

America’s longest wars are finally ending, but politicians from both parties worry about the strange new peril facing the Pentagon: impending automatic budget cuts.

Unless Congress and the White House reach a compromise, Pentagon spending will be slashed by $54 billion on Jan. 2. That could force layoffs of 100,000 Defense Department civilian employees, devastate vast parts of the defense industry, and affect purchases of ships, planes and almost everything else the world’s largest military buys.

That prospect is so politically unpleasant as the nation fights deep unemployment that defense contractors, Pentagon officials and members of Congress say a last-minute reprieve is almost a certainty when Congress convenes in a lame-duck session after election day.

“There’s about a 90% chance it will never happen,” said Gordon Adams, a defense budget expert at American University and a former senior official at the Office of Management and Budget.

That isn’t stopping dire warnings and raw accusations on the campaign trail.

Romney, who vows to boost defense spending, has charged repeatedly that the cuts are the product of a White House plan to weaken defense. The administration, in turn, has blamed the Republican-led House, including Romney’s running mate, Rep. Paul D. Ryan of Wisconsin, for forcing the cuts.

Lost in the finger-pointing: Leaders of both parties agreed to the automatic spending cuts, known as sequestration, in acrimonious and ultimately unsuccessful negotiations last year between the White House and Congress to reach a grand bargain to lower the federal deficit.

They agreed to cut nearly $1 trillion in planned federal spending, including $487 billion at the Pentagon. But they also agreed on the need to trim $1.2 trillion more over the next decade. The White House insisted that half of the savings should come from domestic programs and half from the military, in a deliberate attempt to ensure it doesn’t happen.

Ripping up the defense budget was so unpalatable, the thinking went, that it would force both parties to compromise and reach a comprehensive deal that addresses taxes, mandatory spending on Social Security and Medicare, and other areas of the federal budget.

So far, it hasn’t worked.

“They insisted upon [defense cuts] … in the debt negotiations,” Ryan said in the vice presidential debate Thursday night, repeating Romney’s charge that the White House wants “devastating cuts on our military.”

Vice President Joe Biden shot back that Ryan, chairman of the House Budget Committee, had supported the deal that ordered the automatic cuts on the Pentagon budget.

The most likely outcome is a quick fix to delay the cuts and provide more time to craft a deficit-reduction deal. But the timing, and the final deal, may depend on who wins the White House next month.

“If Obama is elected, I don’t see the Republicans suddenly caving” and agreeing to raise taxes to reach a deficit deal, said Todd Harrison, a defense budget analyst with the Center for Strategic and Budgetary Assessments, a Washington think tank. “If Romney wins, the Republicans may want to wait until he gets into office to make a deal.”

If sequestration occurs, the Pentagon budget would be capped at $491 billion for 2013, down from the $546 billion that the Obama administration is seeking next year for national defense.

The war in Afghanistan doesn’t count against the cap and military pay is also protected. But almost every other account in the Pentagon budget would be trimmed the same — slightly more than 10% — to get down to the cap, Harrison said.

“Having the flexibility to target the cuts would make a huge difference,” he added.

In inflation-adjusted dollars, the Pentagon budget is now the largest since World War II. Most of the money goes to defense contractors for expensive new weapons systems, like the F-35 Joint Strike Fighter under development, as well as to medical and retirement benefits for troops and their families.

The defense budget is so large — more than $610 billion this year — and the annual federal deficit so deep that many experts believe the Pentagon will face further cuts in the eventual budget deal, no matter who wins.

“It’s almost like Newtonian gravity,” said Adams, the defense budget expert. After more than a decade of two wars, U.S. troops are out of Iraq and are leaving Afghanistan, so the Pentagon’s needs will shrink unless another major conflict drives spending back up.

Some defense budget experts think the Pentagon could easily absorb spending cuts of 10% or more as the wars recede. But the inflexibility of automatic cuts would be harsh for the military and the defense industry, which has grown accustomed to lavish budgets.

Industry executives have tried to pressure Congress and the White House, warning that the sequester would require widespread layoffs at defense plants around the country. Other experts say layoffs probably would be gradual because most funding on major weapons contracts is paid out over years.

david.cloud@latimes.com