Rep. G.V. (Sonny) Montgomery (D-Miss.), a retired National Guard major general who chairs the House Veterans' Affairs Committee, says he's engaged in guerrilla warfare, complete with hidden opponents and captured documents. His fight is to keep the New GI Bill alive.

The little-noticed conflict pits Montgomery and the military services against Defense Secretary Caspar W. Weinberger, who usually champions their causes; Congress against the Office of Management and Budget; and scores of unidentified irregulars in the White House, Pentagon and Congress against each other.

"I don't know whether it is the Office of Management and Budget, the secretary of defense or the president himself who has decided that we should repeal the New GI Bill," Montgomery told House colleagues last week. "I do know the decision is seriously flawed."

The New GI Bill became effective last July 1 and is scheduled to run through July 1, 1988, as an experiment to see how much it helps recruitment. But the OMB has notified Congress that, as an economy measure, it will propose legislation to abandon the experiment and return to the system that preceded it.

Ever since World War II, the federal government has offered some package of educational benefits in exchange for service in the military. The term GI Bill is an umbrella covering all these educational assistance programs.

Under the New GI Bill, $100 a month is taken out of a recruit's pay during the first year of service and the government sets aside $200 a month. In subsequent years, the government sets aside $300 a month. This totals $10,800 during a typical three-year hitch, of which $9,600 comes from the government and $1,200 from the GI's pay. On becoming a civilian, the former GI is paid $300 a month by the Veterans Administration for three years to help finance college or technical training. The legislation also authorizes the services to set aside an additional $400 a month for personnel performing critical skills, such as electronic and boiler technicians. The law also covers education pay for reservists.

Under the previous law, called Veterans Educational Assistance Program (VEAP), the GI could lay away a maximum of $2,700. The government would match it 2 to 1, or up to $5,400, making a total of only $8,100 the veteran could use for college. VEAP also authorized additional payments for people with critical skills.

"I don't know how to fight this thing," Montgomery said in an interview. The real opponents, he said, seem to be hiding in the bureaucratic jungle and forcing military leaders to spout a line they do not believe, even deleting favorable remarks about the New GI Bill from testimony prepared for Congress.

In submitting the fiscal 1987 budget to Congress, the OMB said that terminating the New GI Bill on Oct. 1, 1986, rather than July 1, 1988, and going back to VEAP would save millions. The more generous program is not needed, the OMB contended, because the services are having little trouble filling their billets with high-quality people. The Veterans Administration, in its own budget request, played what Montgomery said was the role of good soldier, declaring, "the Department of Defense has determined that the New GI Bill is not necessary for recruitment and may be a disincentive for retention."

Whoever wrote that, Montgomery said, did not know what he was talking about, especially in saying that the New GI Bill is not needed to entice high school graduates into the services. The congressman displayed a pile of Pentagon documents, some of which the services must wish he had not captured, to buttress that contention. Samples:

*Army . . . "Switching back to VEAP after completing the costly conversion to and implementation of the New GI Bill will . . . cause damage to the recruiting program in general and the Army's educational incentives programs in particular," according to an internal document opposing the OMB plan. "The GI Bill provides for equity in America. It is a low-cost, highly visible means for economically deprived youth to finance a higher education and fully achieve their potential. New soldiers in the Army are participating in the New GI Bill at rates dramatically higher than the previous education program. The current participate rate is 68 percent . . . . " Secretary John O. Marsh Jr. wrote Weinberger during the Pentagon's budget deliberations that terminating the New GI Bill "will take from the Army an important and efficient tool in the recruiting of a quality active and reserve force."

*Navy . . . Secretary John F. Lehman Jr., in recent testimony before the House Armed Services Committee, said of the New GI Bill, "It is a great program, and you are to be congratulated on it." Ironically, the evidence he cited was that the Army had made a greater effort than the Navy to use the bill to attract volunteers. The result, said Lehman, is that "for the first time in many years" the Army, not the Navy, is the first choice of high school students.

*Air Force . . . An internal memo said suspension of the New GI bill would have a "serious impact" on recruiting quality people and "would send clear signal to youth population that services do not care about their educational development. Request that the New GI Bill be permitted to continue through the life of its originally legislated three year period."

The OMB proposal has little or no chance of being approved in the House, congressional aides said, but might be added to the Senate bill authorizing Pentagon spending in fiscal 1987. This would make the House-Senate conference on rival authorization bills the battleground for preserving the New GI Bill or going back to VEAP.

Montgomery said he has talked to Weinberger several times but has been unable to move him away from the OMB position. Nonethless, the Mississippi general said he has drawn a line in the dust: no legislation to kill the New GI Bill will get out of his committee, and he is sworn to win this guerrilla war.