Boston, Mass. — Shawn Johnson is still the gymnast the world is watching — and envying.

Johnson, 16, the world champion from West Des Moines, nailed down her second straight national championship Saturday with power and precision.

She completed her difficult vault with 21/2 twists for the second time in the meet at Boston University’s Agganis Arena, and finished with a dynamic new floor exercise to “August Rush” music to total 127.50 points. Her score was the highest at nationals since a new gymnastics scoring system began in 2006.

“I just have more confidence,” said Johnson, relieved to have landed her vault in both Thursday’s first round and Saturday’s finals. “I’ve landed it in competition. And I have that behind me. I know in my mind that I can do it again.”

Johnson will be a favorite in the Olympic Trials on June 19-22 at the Wachovia Center in Philadelphia. The top two all-around finishers there automatically make the six-woman team competing in Beijing.

The 4-foot-9 Johnson became the eighth repeat U.S. champion. Former Olympic medalist Kim Zmeskal is the lone winner of three straight titles (1990-92).

Two-time champion Nastia Liukin of Parker, Texas, was the runner-up with 126.50 points. Liukin, who will challenge Johnson for Olympic gold, climbed back in the race with a sensational 17.100 on the uneven bars, believed to be the highest score ever in the world in the event. Liukin scored a 17.050 Thursday.

“I didn’t think of placement at all,” Johnson said. “I just wanted to go out there and do my routine, and seal my routines.”

Johnson scored a 16.20 on her new floor exercise that includes a triple move. Liukin performed on the floor immediately after Johnson and needed a 16.95 to tie Johnson for the all-around lead. Liukin scored a 15.850. Johnson and Liukin had 6.6 and 6.2 difficulty scores on the floor exercise, respectively.

“I thought my floor exercise was great,” Johnson said. “I have to work on my triple a little. I had too much power going into it.”

Johnson led throughout the two-day competition. On Saturday, she had the best scores in the vault (16.00), balance beam (16.30) and floor exercise and tied for third on the uneven bars (15.55). She improved all four scores from the first day of competition.

“I can definitely improve on the little details in every event,” Johnson said. “We’ve got one week at home of hard-core training, and I can’t wait for it.”

Chellsie Memmel, the 2005 world champion from West Allis, Wis., coming back from a shoulder injury, was third.

Johnson, named national gymnastics athlete of the year, has won all but one competition since becoming a senior-level gymnast in 2007 — she lost the American Cup in March after falling on the vault.