Washington — The nation's capital may be shocked by reports detailing a direct United States role in mining harbors and making raids on the ports of Nicaragua. But back home in the congressional districts, many Americans have only a passing interest.
A check of a cross-section of congressional offices and members of Congress found neither an avalanche of mail or phone calls, nor a clear expression of sentiment from constituents on Central America.
''People don't understand what the stakes are,'' said Sen. Alan K. Simpson in a telephone interview from his state of Wyoming. ''The Central America issue remains a conundrum wrapped in an enigma.'' The Republican added that ''Somozas, Sandinistas, Contras, and Contadoras,'' the names associated with Central America, mean little ''when you're trying to run a store downtown.''
Rep. Henry A. Waxman (D), whose Southern California district is geographically closer to troubled Central America, has received only about 25 letters on the US efforts to mine harbors of leftist Nicaragua. Americans are only interested in foreign policy when Americans are being killed abroad, an aide concludes.
Even Rep. George Miller, a Democrat who has made the mining a major issue and whose California district generally showers him with mail, has generated only 10 letters and phone calls on Nicaragua in the past week.
Other districts report receiving no telephone calls and no letters at all.
''I've gotten no mail and only a few questions,'' said Sen. David F. Durenberger (R) of Minnesota, a member of the Senate Intelligence Committee who has objected to the administration's dealings with Congress over Central America. Constituents ''really don't understand Central America'' and are not critical, he said.
One exception to that rule is in the suburban Maryland district of Rep. Michael D. Barnes, a highly political area just outside Washington where the populace keenly follows foreign affairs and the US role in Central America. ''I'm hearing a very strong message that this action is out of control,'' said Representative Barnes, a Democrat, who chairs the House subcommittee on Latin America.
Although both houses of Congress voted to condemn the US role in mining Nicaraguan waters once the action became public, in Barnes's district ''the principal criticism is that what we did last week was nonbinding'' on the Reagan administration, the congressman said.
Meanwhile, some administration backers are generating support for Reagan's policies in districts where residents see an influx of refugees as a real possibility if the Central American nation of El Salvador falls to communist-backed forces.
''I'm giving a lot of talks about how close (the Salvadoreans) are,'' said Rep. Newt Gingrich (R) of Georgia, who guessed that 85 to 90 percent of his audiences support ''doing something dramatic,'' such as mining, to defeat communism in Central America.
''We're sensitive to the (refugee) issue because we've got 2,500 Laotians'' in the district, said Representative Gingrich. The Republican also pointed to the troops, ships, and air fields supported by the Soviets and Cuba in the region, while ''you have American congressmen jumping up'' over US-built air strips in Honduras and ''one CIA ship off Nicaragua.''
His message is well received in his conservative district, Gingrich said, adding that at a regional Chamber of Commerce breakfast where ''there were more people who wanted us to go after Cuba than wanted us to get out of Central America.''
Rep. Bill McCollum (R) of Florida said some constituents question the mining and the Reagan decision to withdraw the World Court's authority to rule on the matter. But the majority support the President, he said.
''What dominates is the reality of knowing what happened in Cuba,'' said the Floridian. Central America ''isn't that far away.'' But Mr. McCollum conceded that this view may not be shared ''as you move into the heartland.''
In fact, it is not shared in faraway Iowa, where voters have long hewed to a more isolationist line. Sen. Charles E. Grassley, the state's Republican senator, said he is finding a number of critics of US policies as he meets with constituents, but fewer than he anticipated.
''Considering the furor, I expected to hear a lot more than I'm hearing,'' he said after a series of meetings in small towns and larger cities. He noted a general concern about America getting ''too deep'' into the troubles of Central America.
Meanwhile in Virginia, Republican Rep. Herbert H. Bateman said the issue has not come up in his meetings with constituents. Although he says ''the people of my district are very supportive of the US taking an active role'' to prevent communist takeovers in Central America, he has received only six letters on the issue of the mining, with four opposing the policy.
Such mixed and weak signals may give Congress little guidance when it returns next week after the Easter recess. Still on its agenda is deciding whether to give the Reagan administration $21 million it seeks for fighting a so-called covert war against the leftist Sandinistas of Nicaragua. And Congress technically can decide whether to give the administration $32 million for military equipment for El Salvador, although President Reagan has already ordered the Salvadorean aid on his own authority.
No congressional action is expected until early May, probably after the Salvadorean elections. By then, some of the anger over the mining and the lack of consultation with Congress may have subsided. However, aid for covert action in Nicaragua remains in doubt.
''It would take a lot of convincing that there would be no more adventurism and if there is, that we're going to be fully informed,'' said Senator Grassley.
The flap over the mining ''could have a salutary effect,'' said Senator Durenberger, if it forces the Reagan administration to sit down with Congress. He also faults the President for failing to explain his Central America policies to the public because it is an election year.
However, if the public continues to show only mild interest in the issues, as this survey reveals, the pressures to win popular support for foreign policy may not be strong, despite the election campaign.