In history's largest union merger, the Retail Clerks and the Amalgamated Meat Cutters formally join today into a brand new union that will instantly become the biggest single affiliate of the AFL-CIO.

The new supermarket-based union will pull together the 740,000 clerks and 525,000 meat cutters, plus assorted other retail workers organized by the two unions, into the United Food and Commercial Workers, or UFCW.

In a symbolic sense, the founding convention of the new union - which will be addressed today by President Carter and other dignitaries from the top echelons of politics, labor, civil rights and the women's movement - points up a gradual but dramatic shift within organzied labor that has been little recognized.

No longer is the spearcarrier for the labor movement a grimy-faced coal miner or a hard-hatted carpenter.He, or she, is more likely to be a government clerk or a grocery checker.

Not only is the 1.3-million-member UFCW the largest union within the 14-million-member AFL-CIO, but the second largest is a public employes' union, the American Federation of State, County and Municipal Employees (AFSCME). The United Steelworkers, the largest only tow years ago, is now third in size, according to AFL-CIO figures.

For the labor movement as a whole, the UFCW will rank fourth behind the Teamsters, the National Education Association and the United Auto Workers, all independent of the AFL-CIO.

While the new union achieves its instant prominence within the AFL-CIO by way of a merger, it also starts with a fast track record in growth - reflecting not only an organizing zeal that surpasses most other unions but a rampant expansion of the service sector of the economy, both public and private.

"That's where all the growth has been - in the service trades and the public sector," said Albert J. Zack, public relations director for the AFL-CIO.

The new union is expected to line up among moderates and progressives within the AFL-CIO hierarchy. Its president, former Retail Clerks president William W. Wynn, describes himself as a "progressive" whose "hero" was the late United Auto Workers president Walter P. Reuther, who was something less than a hero to the federation's more conservative barons.

In copies of an interview that were included in a press packet for the convention, Wynn said he would like to see the AFL-CIO Executive Council be "more liberal" in its policies and added that he wants to see the UAW and Teamsters rejoin the federation.

He also indicated he expects to take a political leadership role by joining with Seafarers President Paul Hall in soliciting support for President Carter within the AFL-CIO, which has voiced little but criticism for the president in recent months. If the unions do go for Carter's reelection, this would put Wynn in the front ranks.

He says his relations with AFL-CIO President George Meany are "excellent" but declines to say publicly whether he thinks the ailing 84-year-old labor chief should stay on or step down. Meany, recovering from complications from a leg injury, had to cancel an appearance at the merger convention, and speculation is again strong that he may step down shortly.

In a sense, the marriage of the two unions is odd, even though a vast majority of their members work almost side by side in retail food stores, sharing common problems and jurisdictional rivalries since their formation in the late 19th century.

The Meat Cutters had a militant, evne radical history, punctuated with violent stockyard organizing battles and long personified by Pat Gorman, a holder of top offices in the union since the 1920s and a fiery exponent of progressive causes within the labor movement. By contrast, the Retail Clerks were rather bland and conservative until fairly recently.

Negotiations for a merger started in the late 1950s, according to Wynn, but ran afoul of personality differences and jurisdictional irritants. The talks got serious only in the early 1970s.

After six years of intensive negotiations, leaders of the two unions unanimously approved a merger and called conventions to sanction it. The Meat Cutters' union formally approved the articles of merger and voted itself out of existence Tuesday, and Retail Clerks followed suit yesterday - setting the stage for today's merger celebration.

As president of the larger premerger union, Wynn, who rose through the Retail Clerks' ranks from an A&P store in South Bend, Ind., to the union's presidency in 1977, automatically becomes president of the new union. Harry Poole, president of the Meat Cutters since 1968, becomes executive vice president. The new union will be based here.

In an interview recently, the 47-year-old, soft-spoken Wynn said the union will make orgainizing one of its top priorities, concentrating on its supermarket base but fanning out also to department stores, banks, health care facilities and related services. He sees the retail work force as a potential organizing pool of 9 to 10 million members.

The Retail Clerks, with more than 200 organizers, already has one of the best organizing records in the AFL-CIO, although it has to run fast just to stay even because of job losses due to automation.Last year the union took in 50,000 new members, for a net gain of 15,000. CAPTION: Picture, Clerks President William W. Wynn becomes head of 1.3-million-member union. AP