When historians set out to chronicle the mellowing of American political debate that seems to be a hallmark of the middle 1970s, the fate of the federal legal aid program should provide the ma textbook example.

Born amidst the fervor of Lyndon Johnson's war on poverty, and buffeted for a decade by sharp political controversy, legal aid came up for reauthorization in the House yesterday in a placid atmosphere of bipartisan approval.

The vote was 267 to 103 to extend for two more years the Legal Service Corp., the quasi-public agency that channels federal funds to some 3,000 lawyers from Maine to Micronesia who represent poor people in civil legal actions.

The Senate is to voe in mid-July on a five year extension; the difference would need to be resolved in a conference.

Of a series of proposed amendments to limit the types of cases legal aid lawyers can bring, the House adopted two.

One prohibits federally funded lawyers from filing school desegregation suits. The other precludes involvement in cases concerning "homosexuality or so-called gay rights."

"The "gay rights" prohibition drew hoots and guffaws when it was introduced by Rep. Larry McDonald (D-Ga.). It was roundly defeated on two preliminary votes. But when McDonald forced a recorded vote, the "anti-gay" amendment passed, 230 to 133.

Legal aid's transformation from hot potato to humdrum stems largely from changes in the structure, staff and priority of the federal program.

It is operated now not by the political and poverty hierarchy of its stormy decade but by an independent corporation headed by a law professor, Thomas Ehrlich, who prides himself on being non-political.

In place of the provocative "law reform" issues that seemed to consume much of their energy in the past, legal aid offices across the country are concentrating on meat-and-potatoes problems such as contested divorces, overdue rent, excessive repair bills and disputed Social Security checks.

"Why shy away from the emphasis on big cases now," says Ehrlich. "We still have some of those, but our chief goal is to see that every one of the 29 million poor people in this country has some access to a help when he has a legal problem."

Ehrlich observes that the legal profession is "at least ten years behind the doctors in arranging mass delivery of services." An important goal of Legal Services, he says, is "to develop new delivery systems without the turmoil the medical profession has gone through."

The legal aid program went through turmoil aplenty of its own in the years following 1965, when the first federally-funded centers opened in low-income urban centers.

By a gung-ho corps of youthful activists, the simple notion that every man should have legal representation was converted into a sweeping engine of social, economic and environmental action.

Their suits blocked freeways, voided state welfare rules, injected low-income housing projects into expensive neighborhoods, and outlawed consumer credit policies of major retail chains.

By 1971, conservatives in Congress and the Administration were furiously looking for a way to kill, or at least contain, the program.

In a compromise with Congressional leaders, then President Nixon agreed to continue legal aid if it were transferred to an independent corporation.

That agreement became snarled in the impeachment politics of 1974, with Nixon at one point offering to kill legal aid altogether as a reward to conservatives who might support him in an impeachment trial.

Two weeks before his resignation, though, Nixon signed the bill, creating the corporation, and the sailing has been mostly smooth since then.

The corporation is governed by an 11-member board, appointed by the President with Senate approval, which in turn hired Ehrlich - who left the deanship of Stanford Law School to take the job.

Ehrlich, intense but friendly, is responsible for funnelling about $200 million annually in federal funds to about 300 independent legal aid programs.

The average corporation-funded law office has six to ten lawyers operating from a storefront in a low-income neighborhood. For free assistance, clients must fall within a locally determined definition of "poor," which generally means proving that the family income is under $7,000 annually.

The staff attorneys tend to be young - with salaries averaging about $12,000 per year, it is hard to attract experienced lawyers - and they don't stay very long.

Despite Ehrlich's chosen emphasis on routine work, federally funded lawyers continue to bring some precedent-breaking cases.

The ongoing Indian land claim against the state of Maine is the work of a legal services office. Last month's Supreme Court decision permitting a Cleveland woman to live with her grandson, despite a local zoning ordinance to the contrary, was won by a legal aid attorney.

The corporation also funds 13 research oriented study groups around the country to review new approaches in various areas of the law.

In the old days, when they were known as "back-up centers," these research arms stirred particular ire among congressional conservatives. But Ehrlich has changed their name to "support centers" and toned down the nature of their work.