MORENO VALLEY, CALIF. -- Roger Nash, a 31-year-old welder at an aerospace plant, leaves home at 4:15 a.m. for his seemingly endless, 66-mile commute over the clogged Riverside Freeway. His wife, Donna, 29, and their four children stay behind, contending with crowded schools and new neighbors in this dusty valley so different from the Los Angeles suburb of Lynwood they left six months ago.

They would not have it any other way.

Sprouting here on rocky desert four miles east of Riverside is Moreno Valley, the fastest-growing city in California and one of the last places in the state where such a young family can rent a four-bedroom house with trees and lawn on a quiet street for less than $800 a month.

Across from their white stucco home with light brown trim, ground has been broken for an apartment complex as the new city, incorporated in 1984, marches northeast toward the Badlands Hills. Street maps are obsolete almost from the moment they are drawn. Housing permits for 20,000 more units have been approved. Highway 60 is thick with signs promising new malls, subdivisions and this telling advisory: "MORENO VALLEY NEXT 8 EXITS."

Growth has been so rapid here and in other Riverside County communities, including booming resorts and retirement centers around Palm Springs, that California's 37th likely is now the nation's most populous congressional district.

With more than 900,000 residents, nearly enough for two districts, Republican Rep. Alfred A. McCandless's constituency 60 miles east of Los Angeles dramatizes the problems and political potential that attend rapid growth in the Sun Belt.

The young families struggling with long, clogged commutes would like to see more legislative energy focused on their problems. The business community trying to build local industries hopes that the 1990 census and expected reapportionment by 1992 will provide it more attention.

And some political workers, bone-weary from serving far more constituents than they ever dreamed of, would appreciate a respite. Signy Ellerton, McCandless's administrative assistant, noted the staff has not increased significantly during a decade in which the district's population nearly doubled. "You know those cartoons where people's eyes are bugging right out of their heads?" she asked. "That's it right here.

"We get 1,000 letters and postcards a week, sometimes 2,000," she said. "The two district offices are taking 500 to 600 calls as well, problems you have to spend time dealing with."

Some strains are of the residents' making. The Moreno Valley Unified School District begged for approval in April of a bond issue to help finance construction of 20 new schools. The 56 percent vote in favor fell short of the required two-thirds, forcing the district to rely on year-round sessions in some schools and on trailer classrooms, 353 of which are in use.

Budgets in Moreno Valley, which includes many retired people, are tight, and stretching the paycheck is an obsession. McCandless said he receives considerable mail about the federal deficit, showing widespread concern "about the financial path the country seems to be proceeding down."

Betty Wittemore, who bought a four-bedroom house for $112,000 in 1988, said she and her husband, Eric, "came here for the affordable housing. That was it." Her monthly mortgage payment is $1,200, compared with the $1,500 that she would have had to pay for something comparable 55 miles away in Orange County where Eric works as a nuclear-medicine technologist.

According to the Moreno Valley Business Annual Report, "Nearly 20 percent of Moreno Valley people spend less than four waking hours at home." The report said the majority travel more than 20 miles to work, including 28 percent whose job is at least 50 miles away.

Some residents have put an initiative on the city ballot next month to limit future growth, and the business community is fighting it vigorously. "The free enterprise system does not work without growth," said Mike Neufeld, executive vice president of the Moreno Valley Chamber of Commerce. Even if building were contained here, demand for more housing likely would create another population surge elsewhere in the desert.

Growth has been so rapid that political analysts can only guess about the impact of reapportionment. The Census Bureau has not estimated the population of individual congressional districts since 1986, when McCandless was said to have 700,000 constituents, exceeded nationally only by districts in the Phoenix, Austin and Dallas areas. Growth in all three is thought to have slowed compared with here.

The 37th and its neighboring districts provide a guide to two basic styles of reapportionment. McCandless's 37th, which generally follows the boundaries of Riverside County, and Republican Rep. Jerry Lewis's 35th, largely defined by San Bernardino County, have the cohesiveness and respect for local government boundaries that Republicans hope to make mandatory in two reapportionment initiatives on the state ballot next month. That would be to their political advantage.

Democratic Rep. George E. Brown Jr.'s 36th, winding to encompass parts of the cities of Riverside and San Bernardino and heavily blue-collar suburbs such as Rialto and Fontana, reflects the free-form redistricting favored by his party.

According to Thomas Hofeller, redistricting director for the National Republican Congressional Committee, the three districts have enough people for 4 1/4 districts. One logical solution, he said, would be to halve McCandless's district, with half encompassing the mushrooming Riverside suburbs and the other the rest of the county to the east. But in studying the history of reapportionment, Hofeller has concluded "wherever it will most logically go, it won't go there."

Demographers and politicians may never catch up with the march of humanity into the desert. Even the current drought does not appear to have had much impact. "The real estate prices are getting too high here already," said Carol Bowman, looking out the door of her $130,000 house toward Badlands Hills. "For affordable housing, you have to go further out."