Don Bailey represents the gritty steel-mill towns east of Pittsburgh. His resume boasts he is "the most highly decorated Vietnam veteran in the House of Representatives."

Fifteen months of combat duty: a Silver Star, two Bronze Stars with V for valor, an Army Commendation Medal with V for valor, an Air Medal, a Bronze Star for meritorious achievement and an Army Commendation Medal.

Even now, Bailey is a combative man. In an interview, he shouts rather than speaks. He pounds a hefty fist on the desk. He rises to his feet and waves a memento he took from the corpse of a Viet Cong major. Yes, Bailey sees parallels between El Salvador and Vietnam. "If you ask me personally, I think we should fight," he said.

But in second-term Democrat Bailey's district, they don't see it that way. "There's little or no sympathy for American troops' being involved," he said. That is why Bailey and other members of Congress, no matter how anti-communist, are busy reassuring constituents they are not about to send American boys to fight Marxist guerrillas in El Salvador.

"I'm a practical politician," Bailey said. "You shouldn't get involved in a fight you're not going to see through. People don't want the country caught up in that kind of divisive thing again."

Rumbles of a war are giving Congress the jitters.

The House last week voted 396 to 3 to urge the administration to press for "unconditional discussions" between the guerrillas and the Salvadoran government.

A group of 104 House members wrote President Reagan urging him to accept Mexican President Jose Lopez Portillo's offer to help negotiate an end to the Salvadoran war. Another group of 39 warned Reagan that a decision to allow U.S. military advisers to carry M16 rifles would be subject to congressional review under the War Powers Act.

In the Senate today, Minority Leader Robert C. Byrd plans to introduce an amendment to the act that would require Congress' concurrence before combat troops could be sent to El Salvador.

Thursday, a House subcommittee will begin markup of a resolution calling on the administration to reverse its certification that the Salvadoran government has made progress in controlling civilian murders by military police. Although the resolution, which would bar additional military aid, is given little chance of final passage, it is an augury of battles to come over new military aid requests expected to be sent to Congress within a few weeks.

For most of the country, the economy--high interest rates, unemployment, deficit spending--remains the overriding issue, eclipsing foreign policy. Many members of Congress find little interest in El Salvador in their districts.

Others, however, report a growing number of letters and questions in town meetings in the past month, especially in Catholic areas, where the 1980 murders of three American nuns and a lay worker have left lasting resentment.

"A lot of the opposition to military aid has to do with the churches," said Rep. Millicent Fenwick (R-N.J.). You can tell, she said, because "a lot of letters are dated Monday."

Fenwick, a member of the House Foreign Affairs Committee, opposes more military aid because the Salvadoran military "haven't learned a lesson. They've got to stop this business of dragging a husband and a 14-year-old son out into the street and murdering them. I can't stomach it."

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) sent a delegation to El Salvador last month. O'Neill, whose aunt was a founder of the Maryknoll order that has been active in reformist causes in El Salvador, said he favors negotiations with the guerrillas. The administration opposes negotiations, calling instead on Marxist factions to participate in the March 28 elections.

O'Neill is taking a wait-and-see attitude toward Reagan's expected requests for $35 million in additional military aid this year, and $55 million for fiscal 1983.

Majority Leader James C. Wright Jr. (D-Tex.), however, whose support for Nicaraguan aid in 1980 was used against him in the last election, supports military aid for El Salvador and has told Democrats privately that he fears the party will become "McGovernized" if it opposes further assistance.

Some Republicans, on the other hand, are worried that the administration's bellicose rhetoric could backfire in their own campaigns this November. "The last month has seen a very profound shift in American public opinion from indifference to apprehension," said moderate Iowa Republican Jim Leach. "The administration's approach is jeopardizing the Republican Party in the fall . . . . Republicans would like to see the issue go away, but the Democrats could exploit it. This is a potentially explosive partisan issue."

Leach was a principal author of the letter to Reagan this week on the Lopez Portillo initiative. "I've never had a town meeting where El Salvador wasn't raised," Leach said, adding that "Catholic church activism is getting extraordinary."

Pennsylvania Republican James K. Coyne, who visited El Salvador last month under the auspices of the Unitarian Universalist Service Committee, said, "The foreign aid issue doesn't get you a lot of votes."

When Coyne returned, decrying human rights abuses and announcing opposition to increased military aid, his Democratic opponent, former representative Peter Kostmayer, was quoted in a local paper to the effect that, "With an unemployment rate of 11.2 percent in the industrialized areas of Lower Bucks County, it may have been better to take a field trip to U.S. Steel."

Ohio Democrat John F. Seiberling, author of the letter to Reagan on the War Powers Act, is committed to fighting additional military aid until the Salvaldoran government improves its human rights record. However, Seiberling, whose son served as a Mormon missionary in El Salvador, says he has received virtually no mail from his district on the subject.

"All I'm hearing about is the economy," he said. "General Tire is closing its Akron plant."

On the other hand, David E. Bonior, who represents blue-collar auto workers in the Detroit suburbs, said, "The primary concern of my constituency is the economy; but close behind is this El Salvador issue. A lot of people are worried about their kids fighting a guerrilla war. Vietnam is fresh in their minds."

Bonior held a news conference Thursday with Leach to announce their letter to Reagan on the Lopez Portillo initiative. Afterwards, he said that while he had visited Central America every year for the last five years and had worked actively on the Panama Canal and Nicaraguan aid issues, "I didn't issue press releases."

Now, however, with increasing public concern over El Salvador, "One reason I go after press on this is because I think it is politically popular."

Illinois Republican Tom Railsback, who faces a tough primary next week, has been surprised to find that El Salvador is brought up repeatedly at coffees in his district. He has not made up his mind on the issue, he said. "I'm in a quandary. I'm still searching for what I believe the facts are about the internal conflict."

Confused Democrats are constantly approaching Maryland Democrat Michael D. Barnes, chairman of the Foreign Affairs subcommittee on Latin America, to ask advice on how to respond to constituent inquiries.

At a Montgomery County Chamber of Commerce breakfast two weeks ago, Barnes said, "There were more questions on El Salvador than any other topic, including the economy."

The issue is a difficult one for members of Congress who have often preferred to leave foreign policy to the president. "We're under pressure to support the administration," Barnes said. "But we're getting it from both sides. The left-wing fears another Vietnam. The right-wing is worried about the budget." Gerry E. Studds, the Massachusetts Democrat who is sponsoring the decertification resolution, said, "It is too early to tell what the mood of Congress will be. This is an election year, and the question will be whether members are more frightened of their constituents or of their president."