Members of the nation's largest teachers' union found themselves angry and on the defensive as they assembled here this week to confront the tacks left on their chairs by President Reagan.

The 121st annual summer convention of the National Education Association is expected to draw more than 7,000 delegates and five of the six Democratic presidential contenders. It is the first major gathering of the 1.7-million-member union since the issue of education reform heated up and became the year's most popular political punching bag.

Despite years of pushing for education reform, the NEA is boxed in by one controversial reform issue: proposals to link teachers' pay to their performance. It is a concept embraced by Reagan and, according to opinion polls, overwhelmingly supported by the public. The NEA opposes the idea and has managed to defeat a master teacher plan in Tennessee.

NEA executive director Don Cameron lashed out at Reagan today for his record on education, calling him a "master magician" who has "pulled a merit pay rabbit out of his hat" for political reasons.

The NEA will not engage in the kind of "political opportunism" indulged in by Reagan and a rival union, the American Federation of Teachers, Cameron said. "We are not for merit pay. It has never worked in the past."

Still, in the face of intense public pressure, he and other NEA leaders concede that they have begun backpedaling on their adamant opposition to the idea, saying "we'll talk about it" if certain safeguards are included.

Accordingly, Cameron said, there has been no "volatile reaction" among the teacher-delegates against Democratic candidates who support merit-pay plans.

"It's a hot political ticket. They're running for office. They have the option of throwing themselves in front of a train" or getting aboard, he said.

The NEA will seek at this week's gathering to be "as accommodating as we can to some elements of the master teacher program while still maintaining our strong opposition" to less desirable merit-pay plans, he said.

Former vice president Walter F. Mondale, widely viewed as the Democratic presidential candidate most likely to win NEA support, drew applause here with his praise and sympathy for educators and attacks on the president's record as "the most anti-education in American history."

"I want to be the best president American education ever had," Mondale told the delegates. But he also reaffirmed his recently stated support for merit pay with certain qualifications.

"Everybody's sloganing merit pay now," he told the Pennsylvania delegation. "But that can mean anything; it's been used to pay white teachers more than blacks. The question is how we go about it."

Later, at a news conference, he added: "If it means teachers of greater skill and competency will be paid more, then I'm for it."

Any plans for merit pay should first address the critical need for across-the-board pay increases for teachers, he said.

Many delegates here say they feel threatened by, and are opposed to, any merit pay plan. Buttons reading "Merit Pay--Not Today" were worn on some lapels.

Objections to various merit pay and master teacher proposals center on the possibility that they will be too subjective, demoralizing or political.

The four other Democratic presidential hopefuls expected here to bid for the NEA's endorsement are Sens. John Glenn (Ohio), Gary Hart (Colo.), Alan Cranston (Calif.) and Ernest F. Hollings (S.C.).

Each recorded a 10-minute videotape, as requested by the NEA, which interested delegates can view.

The NEA has set Oct. 3 as a target date for deciding whom to support.