Now that the nation officially numbers more than 300 million, what next?

What will 400 million look like?

If demographers are right, we'll hit that mark by 2043. They and other futurists envision a typical American neighborhood that year will be something like this:

More than likely it'll be located in the South or West, despite scarce water resources and gas prices that make $3 a gallon look like a bargain. Barely half of the community's residents will be white, and one in four whites will be senior citizens. Nearly one in four people will be Latino and multiracial Americans will be commonplace.

"We're going to be growing for the next 50 or 100 years, but it's not because of the birthrate," said John Bongaarts, vice president of the Population Council, a nonprofit in New York. "If the birthrate were to drop we'd have a very different future ahead. If we were not living longer and had no migrants we wouldn't be growing at all."

The U.S. will keep getting more racially and ethnically diverse — by 2043, it will be about 15 percent non-Hispanic black, 8 percent Asian and 24 percent Hispanic.

Ideas about race that hold sway now, simply won't then, just as the attitudes of 30 years ago have changed.

For example, in the 1970s one in three whites favored laws that barred marriage between blacks and whites; in recent years it's barely one in 10.

More than 7 million Americans reported in Census 2000 that they were multiracial — 42 percent of them were under age 18.

"The racial lines will basically be blurred," said William H. Frey, a demographer with the Brookings Institution. "It's hard to say what the different classifications will be. ... The stark racial categories now won't hold."

Mixing and melding will be the norm for today's children, who by 2043 will be moving into positions of power across society as the last baby boomers close in on 80.

"Think of the electoral base," said C. Matthew Snipp, a sociologist and demographer at Stanford University. "It seems likely that the power structures will change."

Demographers say some of today's trends will continue: Rust Belt cities like Detroit, Pittsburgh and Cleveland will probably keep losing population, though some argue that lower costs of living may attract people who can telecommute to jobs elsewhere.

The fastest growing states will continue to be Nevada, Arizona and Florida. Census projections through 2030 show the Sun Belt continuing to gain population.

With some cities and suburbs becoming more densely populated, far-out exurban areas will keep growing — which will probably mean longer commutes and more demand for gasoline. Demographers predict costs for gas and water, now relatively inexpensive, will mushroom.

Lifesaving drugs and technologies will help Americans stay alive longer than ever — and the nation overall will age.

In 2000, 12.4 percent of Americans were aged 65 and older — but that percentage is projected to jump to 20 percent by 2043. More than one in four residents of Florida, New Mexico, North Dakota, Maine, Montana and Wyoming will be over age 65.

Here's another way to think of the senior boom: Between 2000 and 2050, the group of Americans who are 85 and older will nearly quadruple to almost 21 million.

The good news is this will help revitalize rural, retirement-friendly places with lots of natural amenities like the nation's Western mountains and some Great Lakes areas, said Kenneth Johnson of Loyola University-Chicago. "These tourist and retirement destinations are the fastest-growing rural areas," he said, adding that this is attracting workers — many new immigrants — to build houses and tend hotels.

But a big bubble of elderly Americans also will strain Social Security and Medicare, and there will be "big battles" over how to pay for them, Bongaarts said.

Demographers repeatedly warned that projections are iffy — things change.

Expected medical breakthroughs may not happen. World events — wars, diseases, economic ups and down — can stop or speed up immigration. Americans could stop having enough children to replace themselves, which they're just barely managing now. Things that seemed a lock just a short time ago can be thwarted.

Two years ago, for example, California officials downgraded by 15 percent their predictions for state growth, mainly because Latino families were having far fewer babies than expected. When the U.S. hit 200 million people in 1967, the nation was supposed to reach 300 million before the end of the century.

"Nobody really knows for certain where this will go," Snipp said. "All this is premised on many, many assumptions."