Washington — Earlier this year Rep. Jim Leach traveled to El Salvador and Nicaragua, visiting the fighting areas of these troubled countries and talking with officials and churchmen.
That firsthand view may not make him equal to a State Department expert on Central America, but in the view of the Iowa Republican, it comes close. Even the Central Intelligence Agency can't obtain information more directly than can congressional visitors who have recently been going to the region in a steady stream, Mr. Leach argues.
Those trips are part of what he calls the ''democratization of foreign policy'' in the United States.
Ever since the Vietnam war, Congress has kept a wary eye on US involvement abroad. Now it is stepping up its vigilance as a range of congressional panels are acting on military aid to El Salvador, secret funds for counterrevolutionaries in Nicaragua, as well as the MX missile.
In recent weeks the House has also attempted, albeit clumsily, to direct arms control negotiations by its nuclear freeze vote, and a House panel has attempted to exclude South Africa from International Monetary Fund benefits.
Once content to stand back while presidents made foreign policy, Congress is stepping out of its passive role. While it always controlled the purse strings, and the Senate could vote on treaties and confirming presidential appointees, Congress now wants a role in forming policy.
''It's a new foreign policy phenomenon,'' says Leach, a member of the House Foreign Affairs Committee, who cites the House freeze resolution that calls on the US and Soviet Union to halt the arms race and negotiate a permanent arms agreement. That vote goes beyond approving or disapproving a finished treaty. It aims at telling the administration how to negotiate a treaty that doesn't yet exist.
Foreign policy used to be left to an elite group of specialists on Capitol Hill, says Sen. Dave Durenberger, but ''now everybody is grinding out papers'' on the subject. ''It's because the people of this country are getting much more involved in these issues,'' according to the Minnesota Republican who serves on the Senate Intelligence Committee.
But even as Congress steps into the foreign policy field, it is stepping gingerly. There are political mines that could explode, especially for Democrats, who are for the most part leading the opposition to the Reagan policies.
Foreign policy critics among Democrats maintain that the President is sending too many guns and not enough butter (or human rights enforcement) to El Salvador. But they don't want the blame if El Salvador should fall. ''Tip has that concern,'' says an aide to the House Speaker Thomas P. (Tip) O'Neill Jr. of Massachusetts.
So far, the result has been caution. The Democratic-controlled House Foreign Affairs Committee this week fashioned a new El Salvador military aid program that would give the Reagan administration most of the money it seeks, while attaching a new requirement that the El Salvador government engage in ''dialogue'' with its opposition.
The proposal would also require El Salvador to set human rights objectives, and would permit either house of Congress to cut off military funding to the country for fiscal year 1985. Meanwhile it would all but ensure funding for more than a year.
In similar fashion, Congress and the Democrats have avoided taking absolute stands against the MX missile. A House subcommittee on the same day released funds for testing the MX, following a conciliatory letter from President Reagan that promised more flexibility on arms control.
Even the nuclear freeze resolution, at first predicted to win an easy victory because of grass-roots support, was watered down during seven weeks of off-and-on debate in the House. And a Senate version would probably be even weaker.
In further action, the Senate Foreign Relations Committee voted Thursday to cut off aid to El Salvador after 1983 unless President Reagan submits a report to Congress on El Salvador's plans to stop the killing of civilians by right-wing death squads.
House Democrats have been the most forceful in opposing the Reagan administration's covert aid to counterrevolutionaries in Nicaragua. The House is expected to take up soon a bill to cut off funding for the secret activities and replace it with direct, open aid for interdicting communist arms shipments.
But even the Republican-controlled Senate has expressed displeasure at the secret aid program. Senator Durenberger helped form an Intelligence Committee proposal to require the White House to explain its aims and perhaps face a cutoff. ''We didn't stop the covert action in its tracks,'' he concedes.
He adds of the Senate committee action, ''If I were the President, I'd be worried. It says there's a deep concern out here.''
Observers of the growing congressional activism on foreign policy are divided about its virtues. ''I'm for consultation,'' says Rep. William S. Broomfield of Michigan, ranking Republican member of the House Foreign Affairs Committee. ''But I think this is getting ridiculous. In many cases they are tying the hands of the administration in carrying out foreign policy.''
''Foreign policy demands a certain nuance and constant attention which the Congress is not well suited to provide,'' says the aide to Speaker O'Neill. However, he says that the members are ''reflecting the will of their constituents.''