Although he lives just across the Potomac River in Anacostia, Aaron Moore, a 15-year-old swimming champion at Cardozo High School, had never been inside the Capitol. Suddenly last week, he was called to testify before a congressional committee as an expert witness on the effects of one of President Reagan's budget cuts.

Facing the congressmen on their high wooden dais, Moore felt the familiar butterflies, the ones that flutter around his chest when he steps up to the starting blocks at a big swim meet.

"Everything I wanted to say I had in a little compartment in my head, but when I got to the table it just blew out," he recalled a few days latter. "I took a deep breath and felt a little more confident. Instead of thinking I was I was going to blow it, I started thinking I was going to make it through."

Moore squinted into the blaze of TV lights and told a dozen congressmen on the House Education and Labor Committee about the National Youth Sports Program (NYSP), a federal project that helped 50,000 teen-agers make it through last year. Approximately 700 disadvantaged youngsters in the District took part int he program of training, travel and competition last summer at Howard University and the University of the District of Columbia campuses.

In his latest budget proposal to Congress, Reagan recommended that $6 million earmarked for NYSP from the Community Services Administration be withdrawn from the 1981 budget. The White House reasons that too much federal money is spent on social programs and these funds should be supplied by local communities, not the federal government.

But for moore, NYSP has worked. Three summers in the program have led to a scholarship offer from Tennessee State University.

"Now I can go to college or maybe even take a seat beside you," he told the congressmen.

"If NYSP isn't continued, the kids in Anacostia will have too much time on their hands, because there isn't much to do," he said. "This program has given me determination."

It would cost the federal treasury about $7.4 million ti keep NYSP going nationwide this summer, according to local project leaders. That's almost enough money to buy one 81mm mortar listed in the Department of Defense budget for fiscal year 1982 at $8.3 million.

Unless Congress overrules the Reagan budget proposal, NYSP's funds, with $36,500 budgeted for UDC and $41,000 for Howard's program, will be directed somewhere else in the federal government. Moore and the 700 NYSP participants might find themselves back on the streets with 700 others who were rejected last year because of lack of money.

Sitting in his living room on Sheridan ROAD sE overlooking four lanes of the Suitland Parkway, Moore talked about NYSP. He has been in the program since it came to UDC three years ago, although it began at Howard in 1969.

Four evenings a week during the summer, Moore took a bus to the UDC campus and ate a free meal supplied by the U.S. Department of Agriculture. After dinner, there were what Moore calls "enrichment" classes.

He learned about nutrition, other cultures, poetry, religion and how to fill out a job application. "They would tell us that sports wasn't the only way to get through life," he said.

Moore chose swimming as an evening activity, instead of dance, gymnastics or racqueball, the other diversions offered by NYSP. But since he was beyond the basics, Moore learned water safety and CPR (cardiopulmonary resuscitation.).

Two or three times a summer, the youngsters at UDC's program took field trips. Moore saw "Dracula" on Broadway, the Penn Relays in Philadelphia, and he went skiing in the Poconos in Pennsylvania. Except for a trip to Pittsburgh, they've been his only trips outside the District.

The National Youth Sports Program grew out of the President's Council on Physical Fitness and Sports in 1968. Administered by the National Collegiate Athletics Association (NCAA), the program reaches disadvantaged youths from 10 to 18 years of age with the goal of breaking them out of the poverty cycle.

It takes much more than $6 million from the federal Community Services Administration to run NYSP. The Agriculture Department also contributes $1.4 million for daily hot meals. Another $10.2 million in goods and services was donated by the private sector or local governments, according to NYSP officials here.

The 130 colleges and universities involved in the program contribute an estimated $8.2 million, including use of campus facilities. Private groups and state and local governments added another $2 million. The NCAA administers the program without charge to the federal government.

Yet without the $6 million federal payment from the Community Services Administration, NYSP could be defunct. "Unless the 1981 funds are made available, the NCAA won't be able to operate NYSP this summer," said Steven Inkelis, the association's attorney here.

That's what Moore tried to tell the congressmen, even though he had only a day to think about his testimony. His swimming coach had approached him at 3 p.m. Monday to tell him he was going to Capitol Hill the next morning.