Since 1975, the federal government has spent more than $1 billion to make the homes of poor, older Americans more energy efficient. It has dispatched work crews to mend cracked walls, insulate attics and caulk windows in more than 1 million homes.

It also has spent more than $2 billion to help needy families pay their heating and cooling bills--both through regular payments and emergency aid programs.

But now that may change. President Reagan's fiscal 1983 budget calls for ending the federal weatherization program, reducing energy assistance, and shifting control of virtually all the government's energy aid programs to the states as part of his federalism campaign.

If Congress agrees, the administration says its plan will save more than $500 million in fiscal 1983, trim federal administrative costs by $1 million, reduce "unnecessary" compliance regulations now imposed upon the states and make it easier for the needy to apply for aid.

But consumer groups claim the plan will, in effect, destroy energy assistance programs and force millions of aged and poor Americans to make a difficult choice during winter months between buying food or heating their homes.

"The administration's stance is the equivalent of a declaration of war on the elderly and needy of this country," Rep. Richard L. Ottinger (D-N.Y.) charged recently at a congressional hearing. "President Reagan has declared elderly and low-income Americans to be enemies of economic recovery."

The president's budget would make four major changes in energy assistance. It would:

Eliminate the Energy Department's Low-Income Weatherization Program.

Combine two programs at the Health and Human Services Department--the Low-Income Home Energy Assistance Program, which helps pay energy bills for poor people, and the Emergency Energy Assistance Program, a fund that welfare workers can tap for clients in emergency situations.

Cut home energy assistance funds from $1.75 billion to $1.3 billion, and eliminate the $61 million that the emergency program had provided.

Force welfare officials to count any energy assistance as "earned income" when determining eligibility and benefit levels for food-stamp applicants and recipients of Aid to Families with Dependent Children. This change would save $175 million in AFDC costs and $231 million for the food-stamp program.

The administration claims its budget cuts are justified because "realistic energy prices have eliminated the need for much government spending on conservation," and because "individuals, businesses and other institutions--motivated by rising energy costs and substantial federal tax credits--have been undertaking major conservation efforts" by themselves.

But many members of Congress, like Ottinger, are not convinced. Reagan wanted to end DOE's weatherization plan last year, but was blocked by Congress, which provided the program with a one-year, $144 million appropriation. Rumbles against ending the program and cutting energy assistance, once again, are beginning to be heard.

The weatherization program was started during the 1975 energy crisis to create jobs and help the elderly reduce their energy bills. But it got off to a rocky start because Congress originally required local governments to hire CETA workers to make the repairs. That rule was changed in 1980 after DOE had amassed a three-year backlog of applications.

Since 1977, DOE estimates it has weatherized about 1 million houses, about one-tenth of the households eligible for aid. In December, 1980, the program hit its peak, weatherizing 35,000 homes in that month alone.

The Reagan administration says elderly Americans will still be able to get weatherization help through two other government programs, community block grants from the Housing and Urban Development Department and HHS' home energy assistance program. The administration says states will be told they can use 15 percent of energy assistance funds, about $225 million, for weatherization.

But consumer groups claim Reagan's budget will, in effect, kill weatherization. Block grants from HUD usually are awarded for public projects with widespread community support, not for weatherizing individual homes, they say. It is also unrealistic to expect that HHS low-income energy funds will be available for weatherization, they add, because that program is already being cut.

"One greatly diminished pot is being expected to fund everything," explained Carol Werner, a spokesman for the Washington-based National Consumer Law Center. "There just isn't going to be enough money to go around."

Congress created the low-income program in 1980 to provide regular payments to the poor to help them cope with the decontrol of domestic oil prices. When the program was first proposed, Congress considered giving it at least one-fourth of the revenues generated by the windfall profits tax on oil. That would have netted the program $3 billion in fiscal 1981, but, instead, Congress limited it to $1.85 billion in 1981 and $1.75 billion in 1982. In 1981, HHS paid benefits averaging $225 per year to more than seven million households.

Under the Reagan cuts, benefits would drop to between $85 and $117 per year, consumer groups say, and even lower if the funds are distributed to more households. HHS estimates that 20 million low-income households could be eligible for the grants.

Reagan also has suggested making the program more flexible. In August, HHS Secretary Richard S. Schweiker said, "Where the law provides this department policy discretion, I will pass that discretion through to the states." Schweiker then outlined 13 general guidelines that states must follow to receive the funds.

Reagan's critics claim HHS wants, in effect, to turn the program into "general revenue sharing." "There is no way to insist that low-income households are treated fairly and their needs recognized," complains Werner.

But the administration says it will reduce state reporting and compliance requirements from 32,490 work hours to 16,000. "States can determine who best to serve without federal directives which obstruct efficient and effective service delivery," budget officials say.