When Rep. Robert N. Giaimo (D-Conn.) took over as chairman of the still-fledgling House Budget Committee four years ago, there were few among professional Congress-watchers who had great expectations.

Although the hulky North Haven lawyer had spent 17 years in Congress before taking the budget post, he never had played a very visible role. His reputation had been mainly as a foot soldier. And he knew little about budgetary issues.

Particularly in the wake of Rep. Brock Adams (D-wash.), who had headed the new committee in its first two years, Giaimo seemed to many to be decidedly mediocre. There was muttering in some quarters that he was not up to the job.

This week, Giaimo will preside over the beginning of Congress' long-awaited budget-balancing effort with the key role as the House's budgetary conscience and an enhanced status among his colleagues that belies all earlier skepticism.

Over the past four years, the 60-year-old New Englander has acquired widespread respect as one of the toughest and most effective committee chairmen in Congress, knowledgeable about the issues and a good negotiator, to boot.

Facing a continual splintering of factions within his committee and and often chaotic situation on the floor, Giaimo has rescued the new budget process from defeat several times, to the admiration of friends and critics alike.

Moreover, he's been at the forefront of the recent push to force Congress to cut back on spending, proposing tough new measures to keep outlays down and to tighten the five-year-old congressional budget process.

"He's grown tremendously in the job," says an admiring House colleague who was one of the skeptics when Giaimo took over the budget post. "He's a great politician," coos a younger member. "He can deal with all sides."

It hasn't been easy for Giaimo, who was a late bloomer by any standard. For one thing, his newfound respect has come only in recent years. His first few months as chairman were lackluster and sometimes awkward.

For another, Giaimo too often has been left in the shadow of Senate Budget Committee Chairman Edmund S. Muskie (D-Me.), whose resounding good-guy, bad-guy floor confrontations have made him a folk hero to some Congress-watchers.

But to many, those impressions are overdone.

Although it's true that Giaimo was a slow starter, taking over the Budget Committee chairmanship was his first bigtime congressional post. Before 1977, he never even had held a full-dress, nonlocal press conference.

And Giaimo supporters warn that any comparisons with Muskie should be viewed in perspective. First, the differences between the two chairmen seem more a function of personal style than a matter of effectiveness.

Muskie, more traditionally senatorial, tends to portray budget issues as more starkly black and white, and to take them on in rhetoric-filled public floor fights -- a panache that has given him an image as a swashbuckler.

Giaimo, by contrast, is more the coalition builder and behind-the-scenes man, skilled at putting together support for a package, but not given to flowery speeches. He's also not given to explosive temper tantrums.

Then, too, virtually all sides concede that in practical terms Giaimo has a much tougher row to hoe with the contentious House than Muskie faces in the Senate.

For one thing, Giaimo does not enjoy the bipartisan support in committee that Muskie now reaps under ranking minority member Henry M. Bellmon (R-Okla.). Republicans on the House Budget panel often are hostile and divided themselves.

For another, the House itself is splintered into far more volatile factions, with liberals and conservatives rarely able to agree and unwilling to indulge in the I'll-scratch-your-back tactics that save the Senate from chaos.

When the two houses meet in conference, it's usually Giaimo who paves the way for a break in the stalemate, quietly soothing opposing factions until the various sides finally agree on a compromise.

Although Muskie seems the more intellectual, House backers note smugly that "the end product in both houses is about the same." In truth, the House and Senate recommendations are rarely very far apart.

Giaimo's record on specific issues isn't bad by any measure. Although the chairman has suffered some setbacks in the now-commonplace House rejections of the budget resolution, he also has won some significant victories.

Last autumn, for example, Giaimo doggedly rode herd on squabbling House members during a conference with the Senate on the final budget resolution and deftly engineered a compromise that broke a six-week impasse.

A few months before, he successfully pushed through a sizable cut in the widely criticized $2-billion-a-year small business "disaster aid" program, beating back vigorous opposition from constituent committees.

Other examples abound.

Giaimo has managed to heighten his influence in the Budget Committee by remaining open to all factions, particularly younger Democrats and Republicans, who often are pulling in decidedly different directions.

Last autumn, for example, the Budget chairman endorsed a spending-limit proposal proffered by conservative Rep. Jim Jones (D-Okla.), despite opposition by both the Carter administration and the House Democratic leadership.

And he continually has cajoled the committee's often-dogmatic Republican bloc to moderate its hard-line views enough to become a potent force in committee deliberations. So far, he's had few takers on that score.

Giaimo occasionally has been criticized for being far too good a soldier for some tastes, too willing to serve as a spear carrier for the House Democratic leadership and the White House on budget issues.

To a point, Giaimo is unabashedly a party man, almost instinctively steeped in the loyalty of old-line New England politics. When there's a choice, he'd rather avert public squabbling among Democrats.

But he's also displayed some independence as well.

In early 1977, for example, Giaimo sharply attacked Carter for knuckling under to pressures for increased farm subsidies, warning that the White House could not continue to talk balanced budgets and cave in on individual items.

Later that year, after the Pentagon tried an end run and undercut his budget resolution, Giaimo rasped: "This is not the Georgia legislature . . . . You don't call up from downtown and say, 'Write a budget resolution'."

A serious-minded, yet good-humored man, Giaimo combines a dogged liberalism on civil rights issues with no-nonsense conservatism on spending and the question of continued growth of government.

Fiercely proud of his Italian-American heritage, Giaimo is quick to jump to the defense of minority groups of all kinds at the slightest hint of so much as a slur. But he's also convinced many social programs are a waste.

"One thing about this job -- it has filled me with a lot of unhappiness and disgust at the way the government has done business," Giaimo recalled last week. "There's a lot of squandering. We're not lean and efficient anymore."

In an interview between House-Senate negotiating sessions, Giaimo spoke passionately of some of his frustrations over Congress' handling of budgetary issues and outlined some of his ideas for dealing with them.

"I'm fairly optimistic that we'll be able to report out a balanced budget in committee, and that the members will vote for it" intact in the initial May budget resolution, Giaimo said.

"What I'm fearful of this year is that thereafter they will go their merry way on spending and entitlement programs and wind up in September in disarray," he added.

In Giaimo's view, the "real flaw" in the five-year-old congressional budget process is "that it doesn't have the necessary teeth to get the job done." Budget targets set in the spring are overridden in the fall.

What Giaimo hopes to push through is a procedural change in which the resolution passed in the spring also would direct key congressional committees to trim current spending to meet the just-passed targets.

The chairman also is trying to persuade the leadership to take drastic new action if the appropriations bills Congress passes this summer overstep the spending targets voted in the spring budget resolution.

He wants the speaker to sit on all the new money bills and not send them to the president for signature until the total is pared to the initial May spending target. Budget Committee parliamentarians say the move is legal.

The Budget chairman also wants to take a stab at persuading the lawmakers to trim the cost-of-living increases granted in Social Security and other benefits, which currently are tied to the now-distorted consumer price index.

Although the step was rejected in last week's marathon House-Senate negotiating sessions, Giaimo believes it is important. "The fight has to be made, even if we lost it," he said. The change would save about $7 billion.

For now, Giaimo is determined to try to push Congress into balancing the budget despite the political pain and inevitable bloodletting that may occur, if only because he believes the situation demands it.

"There's a real crisis out there," Giaimo said of the current inflation scare.To those who raise fears that the cutbacks will produce a recession, he shrugs: "It is not written in stone that we always have to have good times."

Despite the past few weeks' budget-balancing fever, it's still not certain that Congress actually will go along with the president's plan. Skeptics point out that the lawmakers have rejected most of the proposed cuts before.

Still, the consensus is that if the cutbacks can be pushed through at all, this is the year to accomplish them, while publich pressure for the reductions is strong. Giaimo almost certainly will face his toughest test.