The nation’s largest veterans groups, angry that lawmakers usually friendly to defense are open to cuts in military spending, are mobilizing their memberships to pressure Congress to preserve veterans’ benefits.

The American Legion, which has 2.4 million members, sent a letter Wednesday to all members of Congress, warning against cuts that it says would gut the U.S. military and compromise the well-being of veterans.

“This country cannot afford further cuts to an already weakened defense infrastructure,” Fang A. Wong, the Legion’s national commander, wrote.

The letter follows a “call to action” issued Tuesday by the Veterans of Foreign Wars to its 2 million members, which raised an alarm about recommendations made to the Joint Special Committee on Deficit Reduction, or “supercommittee.”

“We and our families who have and continue to serve and sacrifice the most need to raise our voices loudly and clearly before the nation’s debt is placed squarely on the shoulders of our military families and veterans,” said Richard L. DeNoyer, the VFW’s national commander.

The VFW said it was reacting to “ominous” letters sent Friday by the leaders of the Senate Armed Services Committee to the supercommittee. Sen. Carl Levin, (D-Mich.) and Sen. John McCain, (R-Ariz.), respectively the chairman and ranking member of the Senate committee, expressed support for proposals by President Obama last month to modify military health benefits and to create a commission to examine the military’s retirement system.

“The VFW is very disappointed in Senator Levin and Senator McCain for not only acquiescing to the president’s proposals, but for suggesting additional ways to further balance the budget on the backs of military members, families and retirees,” DeNoyer said in a statement.

McCain sought to reassure veterans. “They’re very worried and we want to assure them that we will protect present beneficiaries,” McCain told the Associated Press.

But Joe Davis, director of public affairs for the VFW, said, “You either take care of the troops or you don’t, and any recommendation to further reduce whatever slim financial incentives there are is not how you take care of the troops.”

The veterans’ groups are also unhappy with a joint letter sent to the supercommittee Friday by the chairmen and ranking members of the House and Senate Veterans Affairs committees. The letter, while urging that the budget not be balanced “on the backs of America’s veterans,” added that “no constituency better understands the challenge America faces, and no constituency is better suited to, again, lead by example by putting country first.”

The letter did not propose or advocate specific cuts. It recommended that the supercommittee refer to previous efforts by Congress to save money on veteran program spending. “In past times of fiscal restraint, thoughtful and measured areas of budgetary savings found within veterans’ programs have advanced on a bipartisan basis,” the letter noted.