As though they had been 100 percent behind him from the start, delegates of the American Federation of Teachers chanted, sang and cheered for President Carter today.

Carter had his first happy campaign rally since winning renomination, courtesy of about 3,000 members of the teachers union who hid, burned or otherwise made vanish the Kennedy posters they had waved only 24 hours earlier.

Before the president arrived, AFT president Albert Shanker said he anticipated Carter would use his appearance here to make new commitments to education.

Instead, Carter recited his record and hailed the AFT ('a fighting union"), Shanker ("you couldn't have a finer man") and Edward M. Kennedy ("a great man").

Despite the lack of specifics in his speech, Carter was welcomed at least as warmly as Kennedy was Thursday when the senator came to thank this union for effective support during his unsuccessful campaign.

Carter took note of the AFT's political clout.

"I might say, speaking of teachers and students, I learned a lesson this year about the value of an AFT endorsement," he said. The classrooms in which he learned were New York, Rhode Island, and Pennsylvania where Kennedy beat the president in primaries, aided by AFT workers.

After a lukewarm reception from the American Legion Thursday, Carter was clearly delighted by the AFT chants of "We want Jimmy" mingled with choruses of "Solidarity forever, the union makes us strong."

Kennedy urged the AFT to endorse Carter.The union members made clear that despite their opposition to his creation last October of the separate Department of Education and his long-time close relationship with their arch-rival, the National Education Association, they -- like their champion Kennedy -- were not about to bolt the Democrat Party.

They interrupted Carter with applause frequently and gave him standing ovations when he reiterated his opposition to tuition tax credits and his support for the Equal Rights Amendment.

Carter struck what promises to be a main theme of his campaign when he told the delegates of the 568,000-member union that "seldom in electoral history has the choice been so clear."

"Seldom have the views and the commitments of the candidates been so profoundly different. The American people will be choosing not just between two men, not even just between two political parties, but between two paths that lead to two quite different futures," he said.

Carter and Ronald Reagan are as different as Lyndon B. Johnson and Barry Goldwater, he said. In 1964, Johnson crushed Goldwater, who had become identified as an extreme right-wing Republican.

Reagan's proposals of a tax cut accompanied by increased defense spending accompanied by a balanced budget are "pie in the sky," Carter said.

"But the facts must be presented for the American people to understand the fallacious nature of this kind of proposal," Carter said.

The president said he looks forward to several debates with Reagan and is willing to debate "any candidate who might have even a theoretical chance to be elected president."

"The thing I want most of all, though, is a fair two-man debate between myself and Ronald Reagan," Carter said. Clearly he was wishing independent candidate John B. Anderson would be left out.

Carter once again pointed toward next week's scheduled unveiling of his new economic program, but he appeared to have scaled down his hopes for what it can accomplish.

Early this month he told the National Urban League that "millions and millions and millions" of jobs would be created by the new program to revitalize American industry. Today he told the teachers that "hundreds of thousands of new jobs" will be created.

Of education, he said his administration has boosted spending by 73 percent and he has authorized "an interagency study" on how teachers' salaries and working conditions affect the quality of education.