President Carter is flying to Los Angeles Thursday to say thanks to his biggest bloc of supporters in the 1980 Democratic convention -- not the California delegates but those of the National Education Association.

The 1.8 million-member teachers' union has emerged this year as far and away the biggest interest-group force in Democratic presidential politics and a vital part of Carter's struggle for renomination and reelection.

"If it weren't for the teachers and the mayor's," said one senior White House aide, "Carter would be on his way back to Plains."

So it is not surprising that Carter will make the NEA convention the first stop on his cross-country political swing. On Friday, the NEA delegates are expected to confirm the alliance by voting a formal endorsement of the president's reelection.

The NEA board of directors backed Carter for the nomination last September. That endorsement came at the low point for the president in the polls, when he was lagging badly behind Sen. Edward M. Kennedy (D-Mass.), the favorite of NEA's smaller union rival, the American Federation of Teachers, in the fight for the Democratic nomination.

But Carter had just fulfilled his main 1976 campaign promise to NEA -- creation of a Cabinet-level Department of Education -- NEA officials felt the "way to have impact," as one of them put it, was to get into the 1980 fight early.

That decision and the efforts that stemmed from it have paid off in spectacular fashion, for both NEA and Carter. With a few names still trickling in from states that completed delegate-selection just last week, NEA has counted 433 delegates and alternates to the Madison Square Garden convention.

While NEA will have slightly fewer delegates than the largest state, California, (279 to 306), its 255 Carter supporters far outnumber those of any single state.

So far as Tom Donilon, Carter's chief delegate-counter can determine from historical records, the NEA contingent in New York will constitute "the largest representation of any labor union in the history of Democratic politics." It dwarfs that of such politically active unions as the United Auto Workers and the Communications Workers, as well as the rival AFT.

As the source of about one of every seven Carter delegates, the NEA has exerted the kind of clout on the party platform that other interest groups can only envy. Austerity and fiscal discipline may be the hallmark in other areas, but the Democratic platform hails the 73 percent increase in education funding in the past three years and pledges "a steady increase in federal support" of schools, along with enough specific programs to require 2,700 words of description.

A Kentucky teacher, Alice MacDonald, was the chairman of the "human needs task force" that drafted that language, and eight of the 34 platform task force members were teachers.

They were given that strategic position as a reward from the Carter campaign, which acknowledges, in Donilon's words, that "the teachers were critical to our success."

"They endorsed us early, way back last fall," said Tim Finchem, staff director of the Carter-Mondale campaign committee, "and they got an awful lot of people involved in the campaign. They organized themselves in such a way, in terms of educating their people down to the local level, that is impressive. Nobody else comes close."

What is "almost unbelievable," as Finchem said, is that NEA had never even endorsed a presidential candidate until it backed Carter, after the conventions, in 1976. Its political fundraising is puny compared to many other union, business and citizen groups.

Ken Melley, the NEA's political director and coordinator of presidential campaign activities, said the organization's political action committee has raised much less than $100,000 so far -- small-potatoes as PACs go. NEA budgeted only $40,000 for direct costs of its delegate operation, mostly for mailings.

But Melley estimates the organization expended about a half-million dollars in salaries and expenses for the 40 to 50 staff members who spent part of all of their time on politics these past nine months. And state affiliates put in much more.

But the real key to NEA's success was the readiness of hundreds of local teacher-volunteers to spend after-school hours, evenings and weekends doing the kind of organizing work that is the key to influence in the Democratic Party's open caucus and primary system.

For many of these volunteers, like Jean Seeland of Waterloo, Iowa, the motivation was a chance to be a delegate at the Democratic convention in August in Madison-Square Garden.

Seeland, a first-grade teacher, started working for Carter three months before the January caucuses in Iowa, serving on the Black Hawk County campaign committee and helping recruit and train teachers for the precinct caucuses.

So successful was that operation that at the Black Hawk County Democratic convention, 41 of the 96 delegates were educators. Seeland was the second national convention delegate elected from her congressional district, part of the 12-educator contingent that dominates Iowa's delegation of 31 Carter supporters, 17 Kennedy backers and two uncommitteds.

The teachers won their greatest successes in states like Iowa that use the caucus system. With a widely distributed network of members, averaging 6,000 per congressional district, who are well-educated, accustomed to speaking, and quick to pick up the techniques of political infighting, NEA quickly became the most cohesive force in many of the caucus states. As a result, Melley said, it has at least one member in every delegation except the District of Columbia and the Overseas Democrats.

In some states, the success was so great that it embarrassed NEA and created a backflash among local Democrats. When the Oklahoma Democratic convention met in May, it was discovered that 43 percent of the 932 delegates were teachers. Teachers were elected as 14 of the 42 delegates and claimed 10 alternate posts in the addition.

"Our people down there," said one NEA official, "were trying to make a deal to throw some votes to other candidates, but no one else was well-organized enough for us even to make a deal."

Obera Bergdall, the state Democratic chairman, says, "We've never had this kind of participation from any group." A 39-year veteran of state politics, Bergdall says, "There is some backlash from people who say, 'They [the teachers] haven't participated before, and what makes them feel they should be delegates.' But I think if people get involved, they have every right to win what they can."

"I do think," Bergdall said, "They'll represent the education profession and its interests much more than the people of Oklahoma generally."

Because NEA is a new player in presidential politics, most of its delegates -- including Jean Seeland -- will be attending their first national convention.

The organization is being as thorough in preparing them for New York as it was in helping them get elected. Two briefing bulletins have already come out of Washington for the delegates, and later issues will cover everything from rules and platform matters to low-cost restaurants and anti-mugging precautions in the New York subways.

The teacher-delegates are also helped financially by their union. Seeland and each of the other Iowans will receive $400 apiece toward their expenses from their fellow-teacher's contributions to IPACE, the state association's political action fund.

In fund-raising activities at the NEA convention here reach their goal, NEA's political action committee will give each of the 433 delegates and alternates an additional $250 expense money and reimburse 20 percent of the roundtrip air fare to New York, Melley said.

Teacher-delegates who need more help can get no-cost loans from a teacher's credit-union in Pennsylvania, with NEA paying the interest. All this financial subsidy is designed, not just to help teachers participate, but to remind them whom they are representing.

The NEA delegates and alternates will caucus each day in New York, starting on the Sunday before the convention opens. As George Brown, executive director of the Iowa State Education Association, told a recent meeting of that state's teacher-delegates, "Let me give you a small piece of advice. Your first caucus in New Your will be at 3 p.m. Sunday at the Statler, a pleasant 24-bloc walk from your hotel. Be there. Because if you don't show, you don't get your dough."

"In addition to our own caucuses," Melley said, "we'll have our own whip system on the convention floor. The only thing we need is an issue."

The Carter forces count on the teachers as stalwarts in resisting Kennedy's efforts to change the convention rules or rewrite platform planks -- and as an aid in bringing Kennedy backers to Carter's support by the end of the convention.

While Melley can identify only 24 Kennedy delegates within the NEA contingent, he expects most of them to switch to Carter once the balloting is complete. David Somsky, 33, of Sioux City, is one of those Kennedy backers, and he will play the game. "I will stick with Kennedy as long as he goes," Somsky said, "but when he releases his delegates, I have no trouble going to Carter."

NEA's final test will come in the autumn campaign against Republican Ronald Reagan and independent John B. Anderson. NEA is distributing a scorecard on Anderson showing him only 33 percent "right" on education issues in the last six congresses.

As for Reagan, NEA is already targeting him for condemnation. The organization can identify only a handful of Republican convention teacher delegates -- a fact that Melley blames on the collapse of the Anderson and Howard H. Baker Jr. campaigns.

And in mailings to its Democratic delegates, it is saying that Reagan's calls for abolition of the Department of Education, his support of tuition-tax credits for private school students, his view on school financing, and his opposition to the Equal Rights Amendment will make his platform "unpopular among the nation's educators."

What is more, NEA says, Reagan has said NEA "is becoming more and more a sort of union of educators, and I don't believe that is right."

To an organization feeling its political oats, those are fighting words -- an cement for the alliance with Carter that will be celebrated in Los Angeles the next few days.