In the late 1960s it was food stamps: Congress liberalized eligibility and the program grew by billions of dollars. In the early 1970s much the same happened to subsidized housing for the poor.

Now, it appears that a third program -- federal aid to college students -- is gradually climbing by billions and has become an issue in the presidential campaign.

Both President Reagan and Democratic nominee Walter F. Mondale are talking about increasing federal grants and loans to college students.

It is an unexpected twist for Reagan. His administration told Congress in February that student aid would have to be "firmly curtailed" next year.

For his part, Mondale has vowed not to cut student aid.

Last year, the federal government directly provided $7.4 billion to help students go to college, according to the president's Office of Management and Budget.

In 1980, when Reagan was elected, college student aid cost the federal government $4.5 billion. Ten years earlier it cost $600 million. No one knows precisely the percentage of college students receiving federal aid; best estimates put it at about 50 percent of the nation's 7.3 million full-time students.

Government financial aid to college students is offered in loans, grants and work-study programs. As college costs have risen, students have put together several forms of aid to help pay for their education.

Since the 1978-79 school year, costs have risen 47 percent in public schools and 48 percent in private schools, according to the American Council on Education here, an umbrella group for many U.S. colleges universities.

At the same time, inflation has eroded the student-aid dollar, so that even though federal aid is edging up, it has not kept pace with inflation or college costs. In fact, federal dollars make up a declining share of higher education dollars.

The poorest have suffered a "double squeeze." Their incomes are not only buying less, but they also are pushing them just beyond complicated new eligibility formulas for grant aid. In effect, low-income students are being forced toward loans rather than grants.

"We're not talking about grant money," said Chris Toppe, director of research and services for the National Association of Independent Colleges and Universities, which promotes support for private institutions of higher learning. "We're talking about students mortgaging their future."

Department of Education figures for guaranteed student loans and Pell grants, the biggest programs, are 3 million and 2.8 million students respectively. Some students receive both forms of aid -- and others; the government has no accurate unduplicated count.

In a fiscal 1985 appropriation bill just passed by Congress, $7.9 billion is allocated for the Department of Education's college student aid, about 90 percent of the government's aid. The total is $1.7 billion higher than the 1984 appropriation.

In its study, "Trends in Student Aid: 1963 to 1983," the College oard said that aid from all sources is approaching $16 billion annually, with almost 80 percent "generated directly or indirectly by federal programs, compared to less than two-fifths of the relatively small amount invested in student aid 20 years ago."

But, the study adds, "The expansion of student assistance ended in the early 1980s."

Federal student aid began in 1964 with need-based programs, followed in 1965 by educational opportunity grants and guaranteed student loans.

Congress established in 1972 what is now the Pell grant program, to provide money based solely on need. In 1978, however, it not only greatly expanded Pell grants to include middle-income students but also passed the Middle-Income Student Assistance Act, which removed all restrictions from the guaranteed student-loan program.

However, the Reagan administration reinstituted family-income restrictions, thereby eliminating a significant number of needy recipients.

Education lobbyists critical of the administration say that other Reagan initiatives allowed many middle-income recipients to remain while increasingly providing aid as loans rather than grants, making education less affordable for low-income families.

Congress has resisted tightening restrictions and recently increased the maximum Pell grant from $1,900 to $2,100. It also voted $3.3 billion for Pell grants, awarded according to financial need.

In the guaranteed student-loan program, only those families with incomes above $30,000 are subject to a needs test, but Reagan has tried to expand that to cover all students.

Toppe said, "The loan is subsidized, but it's not aid when students graduate owing more than their families make."

At Harvard, students whose families earn as much as $100,000 can qualify for federal assistance in the form of federally guaranteed low-interest loans, which are provided by state and private lending institutions. Because of Harvard's $13,800 annual cost for undergraduates, between 60 and 70 percent of the students there receive aid.

It is the guaranteed student loan program, in large part, that is fueling the trend of higher federal aid. Because of the government guarantee, loans at 8 percent interest are readily available, especially to middle-income Americans. More than 3 million such loans, which average $2,500, are held. High interest rates have poured millions of federal dollars into subsidizing interest on the approximately $30 billion in outstanding loans.

In Senate testimony last February, OMB Director David A. Stockman said student aid and higher-education spending had "escaped the general regime of fiscal restraint since 1981 and must now be firmly curtailed."

". . . Federal support of nearly 50 percent of all students enrolled in institutions of higher learning is more than the nation can afford," Stockman said.

Reagan recently told college audiences that he would increase Pell grants and federally guaranteed student loans to keep pace with higher college costs.

Responding to questions at Bolingbrook (Ill.) High School last week, Reagan said "we're not reducing" student aid and "we're discussing right now" increases in grants.