A hush fell over the crowded House Foreign Affairs Committee hearing room as the young chairman of the Inter-American subcommittee announced, in dramatic tones, that he had "gone to the source" in an effort to get firsthand information about fast-breaking events in war-torn El Salvador.

Photographers scrambled for pictures and a few spectators even "aahed" as Rep. Michael D. Barnes, a second-term Democrat from Montgomery County, milked a moment seldom available to such a low-seniority congressman, building the suspense like a nominator at a political convention before identifying his source as President Jose Napoleon Duarte of El Salvador.

Barnes then revealed that during a 40-minute telephone conversation just before the hearing, the rightist leader "assured me" that his tiny Central American nation is involved in "a political war, not a military war," and that he is willing to meet with leftist guerrillas.

"President Duarte said, and these are very close to his exact words," Barnes said earnestly, "'you're a politician. I'm a politician. What we do best is talk. I'm willing to talk.'"

In just over two years on Capitol Hill, Barnes has demonstrated that he is a politician who has an instinct for timing as well as talking.

Last year, he grabbed the national spotlight by seizing on the fading popularity of President Carter to call for an open convention by the Democratic Party, an idea which in retrospect many of his colleagues say may not have been such a bad one at that.

This year, Barnes again has benefited from someone else's rejection. When Democrats on the Foreign Affairs Committee organized for the 97th Congress, they turned their back on the returning chairman of the Inter-American subcommittee, Gus Yatron of Pennsylvania, and for the first time in 30 years, picked a sophomore representative, Barnes, to head a subcommittee.

"Mike didn't take the initiative," said Keith Haller, his administrative aide, "but he had established good relations with his colleagues, and it was important for the party to have a strong Democratic counterpoint" to Sen. Jesse Helms (R-N.C.), chairman of the Senate's subcommittee on the Western Hemisphere.

More neutral observers say Barnes' selection was less the result of his merit than dissatisfaction with that of his predecessor. Yatron was rejected by a secret ballot vote of 10 to 9, as was the next ranking member in line, Dan Mica of Florida. "Barnes was the beneficiary of their failures," said one staffer.

Whatever the reason, at age 37, Barnes has found himself presiding over the House subcommittee which is at the center of the congressional oversight of U.S. activity in El Salvador. He is besieged daily by requests for conferences with foreign ambassadors, State Department experts and the news media.

Barnes has never been to El Salvador, and has no plans to go there, so to get up to speed on such a complicated area, he has been doing "more reading than since I was in law school (George Washington, 1972)." Each day when he arrives at his office on the sixth floor of the Cannon Building, he finds a neat stack of clippings from newspapers and esoteric journals, compiled by the Foreign Affairs Committee staff and his own subcommittee professionals, Victor C. Johnson and Robb Kurz.

One of last week's sessions was devoted to hearing opposing views from scholars, but the next day, most of the witnesses bolstered Barnes' view that "there is a real possibility that sending personnel there was a mistake. We could have achieved the administration's goal without military involvement."

As he pondered the testimony, Barnes rubbed his hand across a Nixon-like 5 o'clock shadow and leaned backwards in the chairman's chair, permitting ever-present staff experts to keep him one step ahead of the witnesses.

While Barnes insists that he still spends more time on local matters and constituent problems than he does on foreign affairs, he likes to point out that "Montgomery County probably is more interested in international issues than any congressional district in the U.S. I am just staggered by the volume of mail I get on foreign affairs."

Yesterday the hearings resumed and the subcommittee heard testimony from former U.S. ambassador Robert E. White and several Salvadoran expatriates.