Nils Lofgren the diminutive Washington-bred rock figure, is intrigued by the idea of success by elimination. "It's encouraging that a rock 'n' roll artist with a simple format can still reach a lot of people," he says.

Unfortunately, he's talking about his friends Peter Frampton and Bob Seger, who between them have accounted for more than $100 million in record sales over the past three years, after having paid dues by constant touring since the beginning of the decade.In contrast, most of the other bands from that era have ceased to exist.

Lofgren himself has yet to capitalize as Frampton and Seger have, despite following the same exacting path to gradually widening audiences. Tonight he will appear at Georgetown University's McDonough Arena, the largest hall he has worked yet in the Washington area.

Lofgren has somehow managed to maintain his patience. Several years ago, friends and critics alike were put off by his surliness and cockiness, traits more frequently associated with established rock stars, not fledgling ones. These days Lofgren seems level-headed and perhaps even a bit bemused by everyone else's great expectations. Like Frampton and Seger, Lofgren has finally released a live double-album, "Night After Night" (A&M). Whether it will prove to be the necessary spark in an up-to-now inexplosive career remains to be seen.

"But I feel more optimistic about the future than ever before," Lofgren insists. "I think I've learned a lot more than I ever expected. I thought I knew a lot more than I did when I started. It's taken me this long to realize I didn't know anything.

Lofgren arsenal is drawn from a 10-year history in the rock wars though he is not yet 27. It was a decade ago that he first burst onto the Washington music scene with his seminal rock band, Grin. The 10 years before that had been spent in intensive classical study on piano and accordian; the accordian still echoes in the melancholy overtones of many of Lofgren's compositions.

The initial break came in the late '60s when Lofgren teamed up with Neil Young. The Young connection is one of rock's favorite tales, inwhich the brash, supremely confident teenager corrals Young in the dressing room of the Cellar Door and proceeds to sing him a dozen original compositions. Young, perhaps taken aback, was nonetheless impressed. Lofgren ended up touring and recording with him sharing a producer, David Briggs.

In 1974, four records later, with much critical success and meager financial return, Grin broke up with a farewell concert at the Kennedy Center. Lofgren went solo, increased his touring schedule and released three albums marked by increasingly sophisticated, production-oriented material. Throughout, he watched his record sales, double, triple and eventually quadruple to their current level of 300,000 and album.

"The two greatest things in my life are sports and music," Lofgren beams. Gymnistics in particular has occupied his energy; back flips off a minitrampoline have always been a feature of Lofgren's live show.

One song now germinating concerns Sonny Jurgensen. It's called "Go Long." The idea of going long, of sustaining energy, is part of Lofgrine's "athletic rock" concept, where staying power could very wll come down to physical condition as well as mental attitude. He intends to take tapdance lessons when the tour ends and already has installed a sidehorse in his basement.

Three years ago, Britain's presitigious Melody Maker ran a profile on Lofgren as part of a continuing series. The series title? "Underrated musicians of yesterday." Nils was 23 at the time. "Night After Night" may permanently alter tags like "underrated," "critic's favorite" and "unsuccessful." Whether the album repeats the astounding breakthrough of "Frampton Comes Alive" or Seger's "Live" is conjecture. The next few months should tell Lofgren whether or not he himself will have to continue to "go long."