At his best -- and that was very often at Constitution Hall last night -- Elton John solidifies his reputation as one of the great performers of the '70s. What was sad was the undercurrent of feeling that there will be a lot less of Elton John in the '80s.

Many of the ballads set early in the program seemed to be subtly veiled goodbyes. It was almost as if these songs, some 10 years old, were written for this performance. Opening with "Your Song," John stated "my gift is my song . . . it's the best I can do." A little later, he reemphasized, "I've finally decided my future lies beyond the 'Yellow Brick Road.'" And on "Rocket Man" he supported the passion of the evening's later and more rock-oriented songs: "I'm burnin' out my fuel out here alone,' cause I know it's going to be a long, long time."

The performance, to be repeated tonight, had its slow moments. But for the most part it was wonderful to hear John alone at the piano, singing almost all of his best songs the way they were written with lyricist Bernie Taupin, without unnecessary accents or layers. When percussionist Ray Cooper appeared with his Dr. Frankenstein countenance and behavior, it destroyed the empathy John had spent more than an hour establishing with his audience.

Who cares about John's receding hairline or exceeding paunch? Audiences grow old along with performers without altering the value of either the art or the appreciation. Elton John threw off enough sparks last night to keep the home fires burning. He's earned his excesses, though they were surprisingly few in this performance. Few musicians could have held the stage as tenaciouly or as well. Another line from "Yellow Brick Road" goes, "there's plenty like me to be found." We wish, Mr. John, we wish.