NASHVILLE, Feb. 15— Apparently feeling the pressure of trying to win a second consecutive title, 16-year-old Michelle Kwan fell twice and tumbled out of first place after a startling failure in tonight's long program at the national figure skating championships. Her poor effort was countered by a composed, athletic performance by Tara Lipinski, who became the nation's youngest champion at age 14.

After Kwan tumbled to the ice on a combination jump and a triple loop, and also placed both hands down while stumbling out of a triple loop, the 4-foot-8-inch, 75-pound Lipinski skated flawlessly. Performing one of the most demanding routines ever by a female skater, she landed seven triple jumps, including a difficult triple-triple loop combination. The combination, which requires edge jumps instead of a lift-off from the toe pick, apparently had never been landed before in competition.

Lipinski received six marks of 5.9 for technical merit, and 5.9's and 5.8's for presentation. She was voted first by eight of the nine judges. The long program counted for two-thirds of the scoring, and she moved up from second in Friday's short program.

''I was really relaxed,'' Lipinski said. ''I knew what I had to do, and I felt comfortable.''

The night represented a double victory for Richard Callaghan, who coaches both Lipinski and Todd Eldredge, the four-time men's national champion, in Bloomfield Hills, Mich.

Today, Eldredge defeated Michael Weiss, who skated a technically strenuous performance and nearly became the first American to land a four-revolution jump in competition.

Kwan, of Torrance, Calif., will defend her title next month at the world championships in Lausanne, Switzerland, but she will enter the competition without a second American title, for which she was heavily favored. She had won 11 of her 12 previous Olympic-style events and had become the country's most reliable skater. But she had been nervous before this competition, struggling with her triple lutz in practice. She landed the lutz tonight, but unraveled immediately, fell on a toe loop combination, and toppled to second place over all.

''I was a little shocked,'' Kwan said. ''I was standing up and then I was on the ice. I panicked and kind of got a little off balance.''

In another surprise, Nicole Bobek, who had slumped to sixth place in the short program after a season made difficult by a back injury, jumped three spots to third with an airy performance. She also qualified for the world championships, essentially at the expense of Tonia Kwiatkowski, who fell apart tonight after being third in the short program. She drifted to sixth.

''I had nothing to lose,'' Bobek said. ''I just gave it all I got.''

In winning his fourth national men's title today, Eldredge, 25, of Chatham, Mass., did not hit a home run as much as he bunted the runner home from third with a careful, ordinary routine.

It was left to Weiss, 20, of Fairfax, Va., to swing for the fences by appearing to become the first American to land a quadruple jump in competition.

His quadruple toe loop, which was flawed in its landing, elevated Weiss from fifth place in Thursday's short program to second place over all among the men in a breakthrough performance.

It was not until three hours after the men's competition that the United States Figure Skating Association decided, after a conflicting, confusing series of events, that it would not ratify the historical jump because Weiss had touched a second foot down upon landing the quad.

The late ruling did not affect the outcome of the competition. In fact, the nine judges, who gave their scores without the benefit of instant replay, awarded Weiss six marks of 5.9 out of 6 for technical merit.

Morry Stillwell, president of the skating association, watched Weiss compete from the stands at Nashville Arena. Only after watching a slow-motion replay of the quad did he rule that it was invalid because it did not have a one-footed landing.

Although his jump was not validated, Weiss provided the only real excitement among the men on an otherwise clumsy afternoon and gave an intriguing glimpse of the athletic direction in which the sport is headed.

While others struggled through their triple-jump combinations, Weiss landed seven clean triples and the two-footed quad while skating to the muscular music of Carlos Santana.

The quad was first landed unofficially by Josef Sabovcik of Czechoslovakia in 1984, but Kurt Browning of Canada is given official credit for first completing the jump at the 1988 world championships.

''As far as I know, I landed it,'' Weiss said. ''As far as everyone else saw, I skated great.''

Even with Weiss's impressive jumping, Eldredge received first-place votes from five of the nine judges in the long program, which counted for two-thirds of the scoring. While he may never skate with any great passion, Eldredge has dramatically improved his artistry and presentation. While Weiss was the better jumper, the judges apparently thought Eldredge skated a more complete, mature program.

''I thought I skated O.K.,'' Eldredge said. ''Things didn't feel quite right today. But winning a fourth title is great. Not a lot of people get a chance to do that.''

Eldredge did not do a triple-triple combination jump as planned, and seemed wobbly on two jumps, but he recovered with a strong triple axel late in his program, then replaced a double axel with a more ambitious triple toe jump.

His technical marks ranged from 5.7 to 5.9, and his artistic marks were all 5.8's and 5.9's.

''The total package is more important than one jump,'' said Callaghan, Eldredge's coach. ''Michael had a great quad. But Todd had faster speed, faster footwork and better spins. We've been told over and over the complete package wins.''

Photos: Fourteen-year-old Tara Lipinski, after putting on a flawless performance, became the youngest United States champion. (pg. 1); Michelle Kwan, falling to one knee in one of her two stumbles in the long program, had been heavily favored to win her second national title. Todd Eldredge won his fourth national championship. (Associated Press) (pg. 4)