Washington — In an age of slowing population growth, Americans of Spanish heritage are surging ahead. Now numbering at least 14.6 million, Hispanics could become the country's biggest minority group in the next two decades, according to some population observers.
And now, Hispanic Americans are beginning to translate their numerical growth into more clout on Capitol Hill. Only a year ago they could count a mere five voting members of the House. But congressional lines were redrawn to fit the results of the 1980 census, and now nine Hispanics sit in Congress.
''I really feel optimistic,'' says Antonia Hernandez, director of the Mexican-American Legal Defense and Educational Fund (MALDEF) and a chief lobbyist for Hispanic interests.
She points out that even if Hispanics are still only a tiny group among 535 members of Congress, many other members have populations that are more than 15 percent Hispanic in their districts, and by ''political necessity'' these members give her group at least a hearing.
Rep. Robert Garcia, a New York Democrat who chairs the Congressional Hispanic Caucus, underscores that strategy. He estimates that 50 members have substantial numbers of Hispanic constituents - and just to remind everyone, he has ordered a demographic study which he plans to circulate around town.
''There's no question that we're going to have more clout,'' says Mr. Garcia, the son of Puerto Rican parents, in his decidedly Brooklyn accent. (He says his accent brings quizzical looks from Hispanics during visits to Texas.)
Delighted with the results of the 1982 elections, he calls the four new caucus members ''loaded down with enthusiasm and a spirit of wanting to see things happen.''
Among the most energetic is Rep. Bill Richardson (D) of New Mexico, who is pushing the Hispanic Caucus to unite for the first time on legislative issues ranging from water treatment plants to immigration and foreign trade. In part at his urging, the caucus is holding a flurry of meetings and staff conferences.
Mr. Richardson, whose mother is Mexican, outlines his ambitious hopes for Hispanics. First, he would like to better organize Hispanics in Congress. Then he hopes they will develop a legislative agenda and, finally, begin ''establishing ourselves as a force nationally so that we can have an effect on the election of the next president.''
Hispanics have chalked up two important victories. In part because of their strenuous opposition, the last Congress delayed action on an immigration reform bill. They also won an extension of the law providing for bilingual ballots as part of the Voting Rights Act passed last year. During the early stages, even civil-rights supporters held out little hope for the bilingual provisions, but in the end they glided smoothly through both houses.
But the immigration victory was temporary at best. Hispanics in Congress now appear to face an unstoppable demand to control the United States borders. While no one has an exact count, a Senate committee estimated that the number of illegal immigrants was certainly below 6 million in 1978, the latest figure available. About 80 percent of these illegals come from Mexico and Central and South America.
A major reform bill proposed by Sen. Alan K. Simpson (R) of Wyoming and Rep. Romano L. Mazzoli (D) of Kentucky is back again, moving through the committees of the new Congress. Its carrot-stick approach would offer legalization for millions of aliens already in the country, but it would also try to stem the flow of illegals by imposing sanctions on employers who hire them.
It is the ''employer sanctions'' provision that most concerns Hispanics. They charge that it will cause job discrimination against people who merely look foreign.
''Many Hispanics feel that there must be curtailment'' of immigration, says Arnold Torres, Washington, D.C., director for the League of United Latin American Citizens (LULAC). ''But not at the expense of their community.''
Opponents of employer sanctions have so far failed to remove them from the bill, although a House subcommittee has liberalized legalization provisions somewhat.
''We would like to block sanctions,'' says MALDEF's Antonia Hernandez. But she says that the proposal is becoming as ''American as apple pie.''
''Our legislation will never prevent those who wish to discriminate from discriminating,'' Senator Simpson said recently of his immigration reform bill. He added that many Hispanics want a law to legalize the status of friends and relatives who now live here.
''I keep saying, you ain't gonna get legalization without employer sanctions, '' he adds.
Although immigration law is top on the priority list, Mr. Torres complains that ''people think all that we're concerned about is immigration.'' As a lobbyist for LULAC, which claims the biggest membership of any Hispanic organizaton, he says he would like to see Hispanic influence expand into areas such as foreign affairs.
''Our membership clearly opposes the role of the US'' in Central America, says Torres, because of its ''paternalistic'' approach toward Latin America. ''We're not the equivalent of the Jewish lobby (on Israel) yet,'' he says, but he envisions a similar role in the future.
Freshman Rep. Esteban Edward Torres (D) of California, just elected from a district that is 47 percent Hispanic, agrees that there are special feelings among Hispanics for Latin America. ''The Hispanic community is beginning to view with more interest the interrelations with Latin America,'' he says, adding that it is starting to speak out, usually against US involvement in the fighting in El Salvador.
Mr. Garcia sees poverty as the major issue for him as a Hispanic congressman. ''Look, I represent poor people,'' he says. ''Hispanics are for the most part on the lowest rung.''
Hispanics involved in Capitol Hill politics are far from satisfied with their progress. The Hispanic Caucus, for all its plans and meetings, is still in the early stages of organizing. Recently, Rep. Edward R. Roybal (D) of California, the group's dean, introduced his own immigration reform bill to soften the sanctions proposals. However, he moved without first gaining the endorsement of the caucus, and even Garcia has yet to sign it.
But the caucus is learning some lessons, if sometimes the hard way. During the last Congress, it sent an open invitation to colleagues to become honorary members. About 180 signed on, but several ''put their plaque on the wall and then voted against us,'' says Garcia. One honorary member even used photographs of himself next to Garcia and Roybal in his election campaign - against another Hispanic member.
This year the caucus will be more selective in inviting honorary members.
Meanwhile, they still face an uphill trek as they try to make their way toward more power in Washington. ''There hasn't been a big impact here yet that I've seen,'' says one Democratic leadership aide of the Hispanics on Capitol Hill. ''I think they're tending to start to move. As they start to meet together and press their concerns, they'll probably tend to increase their visibility.''