NO ONE QUITE knows when an entertainer officially becomes "an institution" -- but when hardly anyone can remember when he wasn't, he is. And after nearly 75 ever-trouping years in show business, George Jessel, who died Sunday in Los Angeles at the age of 83, was this country's one and only "Toastmaster General of the United States," a perfect rank first accorded him a good three decades ago by President Harry S Truman. That honor, and the friendship of a string of later presidents, merely reflected the national affection for Mr. Jessel -- but what generated this enduring popularity?

A good act, certainly -- rooted, repeated and embellished in the best traditions of heyday vaudeville. So we anticipated, listened and laughed reflexively through those funny/tender phone calls with "Momma," and those endless strings of anecdotes, yarns, up-front patriotism and shameless sentimentality -- all laced warmly with the flavor of his immigrant culture in New York City.

Though the basic Jessel act never changed all that much with the times, he kept on top by taking it to the changing media -- from the nickelodeons of his childhood to the vaudeville circuits, the Broadway stage, films, radio, television and an incalculable number of dinners, rallies, fund-raisers, inaugurals, conventions and special shows for the American armed forces anywhere. At one point, someone did estimate that Mr. Jessel had been traveling about 8,500 miles a week, 40 weeks a year, for about 200 events. He raised more than $100 million for charitable organizations, a record that earned him the Jean Hersholt Humanitarian Award of the Academy of Motion Picture Arts and Sciences.

The Jessel specialty, of course, was the eulogy; Mr. Jessel delivered several hundred of them and even talked about recording his own, to accompany an inscription for his tombstone: "I can tell you here from the shate it is all worthwhile." In a recent interview, Mr. Jessel said, "I don't know what happens after you're gone, but I know something happens.When I come back, I'll phone you."

Hello, Momma . . .