In an era of slick, media-packaged congressmen, Rep. John P. Murtha (D-Pa.), a burly ex-Marine with the pudgy face of a choirboy, is something of a throwback: a political deal maker more comfortable in the back rooms of Congress than on the set of Meet the Press.

Nearly every day, he prowls the floor of the House, putting together deals for the Democratic leadership, the White House, his friends and himself, and adding to his reputation as one of the House's few remaining vote brokers and possibly its premier political operator.

Murtha, 53, holds no leadership post, heads no committee, seldom gives a speech on the House floor and is rarely recognized off Capitol Hill.

But his office is a regular stopping-off point for lobbyists, Reagan administration lieutenants and others seeking favors for their pet projects. "He's a back-room trader," said one admiring Republican official. "His fingerprints are very rarely seen."

For, in a place as consumed by political win-loss ratios as Congress, the handiwork of this amiable former car-wash operator from a depressed steel and coal district in western Pennsylvania is legendary:

*When members of Congress privately clamored for a pay increase, but publicly feared to propose it, Murtha was the one who quietly slipped a few seemingly innocuous lines onto a pending money bill. The amendment had the effect of lifting a cap on the outside income lawmakers could earn.

Opponents didn't know what hit them until hours -- or, in some cases, days -- later. "That was one of his finest hours. That was pure Murtha," recalled one appreciative Democrat recently.

*When a relatively junior member of the House, Rep. William H. Gray III (D-Pa.), was elected House Budget Committee chairman over two more senior colleagues, a Murtha strategy was responsible. He painstakingly put together the northeast-south coalition that won it for Gray and brought Pennsylvania a coveted committee chairmanship.

Said Gray recently: "You will not find Jack Murtha down in the well giving eloquent speeches on policy. But if I had a choice between an eloquent speaker or the strategic technical skills of Jack Murtha, I'll give the speaker to somebody else and take . . . . Murtha and I'll win every time."

*When positions on key committees were doled out by a small selection committee this year, Murtha's candidate for the important committee that writes tax law, House Ways and Means was a little known Pennsylvanian who beat out the candidate backed by the House's second most powerful Democrat, Majority Leader James C. Wright Jr. (D-Tex.).

"Bill Coyne Murtha's candidate was Rep. William J. Coyne (D-Pa.) is not exactly Mr. Personality. That was solely because of Jack Murtha," said one Democrat familiar with the closed-door committee selection process.

Not one to let hard feelings fester, Murtha a few weeks later publicly endorsed Wright's bid to become House speaker in 1986.

*When the Reagan administration and House GOP leadership were desperate to win House approval of aid to Nicaraguan rebels, it was Murtha, a traditional Democratic liberal on domestic issues but a hawk on defense and many foreign policy issues, who helped them fashion a proposal that would garner enough Democratic votes to pass.

*And when hard-pressed steel mills in his district were demanding help, Murtha got his old friend, Rep. Dan Rostenkowski (D-Ill.), chairman of the House Ways and Means Committee, to put a major tax break for the steel industry into a bill that was supposed to raise taxes.

Murtha put together a coalition of 21 votes that Rostenkowski needed for the tax bill. Needing those votes, Rostenkowski slipped in the Murtha tax break -- even though a final version of the bill, without the tax break, already had been voted on by a House-Senate conference committee.

"He loves political intrigue. He likes to deal. He puts the votes together, make no mistake about it," said House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who has long relied on Murtha as his eyes and ears on the House floor.

As a result, lawmakers say, Murtha has as much clout with O'Neill as any member of the House.

"His reputation is, if you're going to put a coalition together, you have to have Murtha," said Rep. Mike Synar (D-Okla.). "That's about the highest compliment you can pay around this place."

Murtha has little to say about himself, and all but flees from publicity. Unlike many of his younger colleagues who flock to television lights, he spends his days hanging over the brass rail at the back of the House chamber and ambling through its private back rooms, patiently wooing his colleagues by listening to their woes, stories, points of view.

True to his Marine training, Murtha rises early to get office work out of the way so that he will have as much time as necessary to patrol the House floor, associates said.

A non-drinker and non-smoker, he doesn't do his work on the party circuit, as many members of Congress do. At night-time social functions, said fellow Pennsylvania Rep. Jospeh M. McDade (R), "He's the first one there and the first one gone."

Murtha declined to be interviewed for this article. "You understand, it would defeat everything I try to do," he said, meaning that a back-room dealer cannot be too overt.

Elected to Congress from the Pennsylvania statehouse in 1974, Murtha took the skills he learned as a Marine intelligence officer in Vietnam -- winning two Purple Hearts -- and put them to work learning the House.

He quickly caught the eye of the Democratic leadership, working with it and the Carter administration early on to pass Carter's energy package.

Murtha was a deft and diligent worker, O'Neill said, willing to put in the hours talking to his colleagues that many other lawmakers are not. As a result, he was turned to again and again, conveying messages, doing favors, learning his colleagues' desires and political quirks.

Over time, Murtha has collected lots of chits, from O'Neill on down. "Make no mistake about it, he'll cash in the chit later," said one lawmaker who recently benefited from Murtha's assistance.

In the process, Murtha has become one of the most astute analysts of his colleagues, able to evaluate quickly how many of them will vote on a given issue, what the sticking points may be and how to approach them to win their support. His position on the House Appropriations Committee, which has the power to fund or kill a program or project sought by a lawmaker or administration, has only added to his store of knowledge.

In the race for Budget Committee chairman, for instance, Murtha helped Gray pinpoint the key committee chairmen and southerners who, in supporting Gray, could swing substantial blocs of votes to him.

Gray, a liberal black from Philadelphia, was counting on votes from the Black Caucus and many northeastern liberals, but, to defeat strong rivals from California and Oklahoma, he needed a broader base.

So he spent several months wooing the southerners in particular, campaigning for many of them among southern blacks and convincing them he wasn't the rabble-rouser that his inner-city liberal credentials might indicate. At the same time, Murtha also approached the southerners to vouch for Gray. Murtha, a conservative on defense issues, has strong ties to many of the southerners.

Finally, Murtha convinced his friend the speaker to abandon his neutrality in the race and back Gray, which O'Neill did at the last minute, to the astonishment of Gray's opponents.

Murtha himself appeared headed for a Democratic leadership post until 1980 when his name came up in the Abscam bribery case. He was shown turning down bribes on FBI videotapes. He said later he met with the fake FBI sheik to try to attract new industry to his Johnstown district.

Murtha was never charged in the scandal and was cleared by the House ethics committee. But lawmakers said that even having his name mentioned hurt Murtha's reputation. It is unclear whether the damage lingers.

Murtha says he would not want a leadership job at this point, because "you can't deal if you're in leadership." As a longtime associate put it, "I think he's largely got what he wants, which is influence."

Murtha's ability to rise in the leadership could also be hampered by his outspoken conservatism on defense issues, which has placed him substantially to the right of a majority of House Democrats. Some liberal and moderate House members are still upset at his stinging speech lambasting the House Democrats for opposing the administration's request for renewed aid to Nicaraguan rebels.

At the same time, the House has changed since Murtha arrived.

The younger, new-breed members who now form a majority of the Democratic Party in the House may not be willing to have an inside dealer in a such a high-profile role.

"It's definitely an old-fashioned, old-style, smoke-filled deal-making," said one of the younger members who admires Murtha's skills, fears him as an opponent, but prefers him in the background.

Still, said Rep. Leon E. Panetta (D-Calif.), one of the new-breed lawmakers who lost his race for Budget Committee chairman this year due to Murtha's maneuverings, "The reality of the place is that for all its reforms it still depends on the John Murthas to make it happen.