WASHINGTON — To hear the Republican leadership tell it, the once-sacred Pentagon budget, protected by the party for generations, is suddenly on the table. But a closer look shows that even as Speaker John A. Boehner and Representative Eric Cantor, the House majority leader, insist on the need for military cuts, divisions have opened among Republicans about whether, and how much, to chop Pentagon spending that comes to more than a half trillion dollars a year.

Those differences were on display Wednesday on Capitol Hill, where the traditional Republican who now leads the House Armed Services Committee, Representative Howard P. McKeon, fought back against proposed cuts in the Pentagon budget even as fledgling committee members supported by the Tea Party said that the nation’s debts amounted to a national security risk.

“I cannot say it strongly enough: I will not support any measures that stress our forces and jeopardize the lives of our men and women in uniform,” Mr. McKeon said in an opening statement that followed up on a letter to Defense Secretary Robert M. Gates urging him not to stop work on the Marines’ $14.4. billion Expeditionary Fighting Vehicle, a combined landing craft and tank for amphibious assaults that Mr. Gates canceled this month.

But Representative Chris Gibson, a Tea Party-endorsed freshman Republican and a retired Army colonel from New York’s Hudson River Valley, made it clear that no part of the Pentagon’s $550 billion budget — some $700 billion including the wars in Iraq and Afghanistan — was immune.