BEIJING — While China has billed the 2008 Summer Games starting Friday as the coming-out party of a new world power, the United States enters the 18-day competition struggling to stay on top both in athletics and on the world stage.

Many observers are predicting a second-place U.S. finish in the total medals count as China's giant sports program mounts a potent challenge to longtime U.S. dominance of the Olympics.

Such a result would be seen by many here as symbolic of shifting global power balances, as China's political and economic star rises while U.S. global leadership wobbles.

This 1.3 billion-person country has quickly become the world's third-biggest economy, behind the United States and Japan, and economists predict it will overtake the United States in three decades if present trends continue.

While the United States teeters on the brink of recession, China is in the throes of an economic boom that has lifted hundreds of millions of people out of poverty.

Chinese officials have been conscious of the symbolism as they spend tens of billions of dollars both to organize the Olympic Games and to win medals in a wide spectrum of sports.

"For many Chinese, the games are almost seen as a sacred ritual," said Susan Brownell, an anthropology professor at the University of Missouri–St. Louis who wrote the book "Beijing's Games: What the Olympics Mean to China." "What's going on in China far surpasses the importance other countries placed on their games."

In 2000, Chinese officials launched a campaign titled Project 119, which identified sports such as track and field and rowing in which Chinese athletes hadn't previously been competitive. The goal was to shoot for many of the 119 gold medals then available in those sports.

That effort has paid off spectacularly, as Chinese athletes have won a growing number of world competitions in the run-up to the Olympics. China ranked second to the United States in gold medals in the 2004 Summer Games held in Athens.

"I clearly expect (the Chinese) to be the dominant team in the Olympic Games for many, many years to come," said Steve Roush, the U.S. Olympic Committee chief of sport performance, in a Wednesday news conference. "We're not used to being underdogs in the Olympic Games. But we'll get used to that."

In June, an analyst with the global accounting firm PriceWaterhouseCoopers predicted Chinese athletes winning 88 medals this month, surpassing the United States by one medal. U.S. athletes also will face stiff competition from a resurgent Russian team that the analyst predicted would rank third.

For U.S. viewers watching the Olympics on television, the games could provide a visual introduction to a modern China that already produces much of the electronics, clothing and other products in U.S. homes, said Gary Kellmann, an American businessman who has written about living and doing business in China.

Those viewers will see a country that has seen nearly 10 percent annual economic growth over the past three decades and has taken over global production lines for whole classes of goods. They'll also see a country beset by environmental degradation, including the layer of smog that has hung over Beijing all week.

"People doing business in China can feel and see the country's rise, but many Americans don't really understand what's going on," Kellmann said. "These games could show them what a lot of us are already seeing."

U.S. athletes hoping to defend their country's medal dominance will be competing against Chinese rivals trained since childhood in special government-run athletic schools that all but take over their lives.

U.S. athletes also will bump up against Chinese nationalistic fervor that has equated Olympic medal performance with the nation's international reputation, said Brian Xie, a reporter with a Beijing newspaper who covers U.S. affairs.

"There's a lot of pressure on the Chinese athletes to win medals," Xie said. "There are also a lot of people here who feel like we're putting too much pressure on the athletes."

The U.S. team, by contrast, is made up largely of amateur athletes who train when they're not working or going to school, said U.S. Olympic Committee chief spokesman Darryl Seibel.

That formula has worked so far for the United States, which has been a longtime Olympic medal leader even against the well-funded athletics program of the former Soviet Union, which China's athletic schools resemble.

The last time the United States was knocked from the top spot in the Summer Olympic medal count was in 1992, when the countries of the former Soviet Union competed as the Unified team and won 111 medals. U.S. athletes won 108 medals during those games held in Barcelona, Spain.

The secret to U.S. success, Seibel said, has been strong college athletics programs, world-class coaches and grassroots support for Olympic sports. Corporate sponsorship has poured millions of dollars into U.S. athletics programs and facilities.

"You have to stay with what works for you, and our system has worked very well for us," Seibel said. "We've made sure that our pipeline of young athletes remains wide and full."

Yet even such confidence comes with disclaimers this Olympics, as China's 639 athletes battle for medals against the 596-member U.S. delegation.

Speaking to reporters Wednesday, U.S. swimmer Michael Phelps, who has won eight Olympic medals, said he wouldn't let himself worry about what other countries' athletes were up to.

"I come in here with a goal I've set for myself, and that's what I'm going to be focused on," Phelps said.

For U.S. Olympic Committee leaders, however, this month's games could mark a sea change, in which U.S. dominance in the Olympic Games comes to an end.

"As economies have become more global and as our world becomes more connected, you'll see the same thing in international sports," Seibel said. "The day when any nation is dominant in international sport is over."