At hearings on the state of the economy, he can drive the chairman of the Senate Budget Committee right up the wall. On the Senate floor, his colleagues say they tend to head for the exits when he gets up to speak.

"He doesn't fill up the chairs too often, except in the cloakroom," said one Republican senator in disdainful tones. "He's so shrill -- almost a common scold."

The senior senator from Michigan, Donald W. Riegle Jr. (D), doesn't seem to mind. His state is in the grip of what many consider a bona fide depression and Riegle says he intends to keep hammering away at the economic misery there until the rest of the country wakes up.

It is a recipe that gives Riegle a high profile back home where he is generally regarded as a shoo-in for reelection this fall. This week, as expected, Riegle was endorsed by the AFL-CIO and United Auto Workers, and, unexpectedly, by the Veterans of Foreign Wars.

Most of the complaints about Riegle in the U.S. Senate come from the Republican side of the aisle. Riegle has his defenders, important men like Senate Minority Leader Robert C. Byrd (D-W.Va.), and Senate Minority Whip Alan Cranston (D-Calif.), who savor his barbs against President Reagan and his policies.

"Some call it strident," said the other senator from Michigan, Democrat Carl Levin. "I call it sustained hardball. He plays the game hard, but I've never heard it said that he plays unfair."

"He is pretty strong when it comes to stating his viewpoint," added Byrd. "He's a good man to have in your infantry, on the front line. He knows how to mold the bullets and shoot them."

Riegle makes no apologies for his approach. He often describes himself as "feeling like a field doctor in the movie 'M*A*S*H.' I never get out of my operating clothes because I have so many damaged cities, damaged companies and damaged individuals coming through the door every day. . . . I have over 700,000 people out of work in my state and one hell of a lot of misery. It's something I really care about and I want to do something about changing it."

At hearings of the Senate Budget Committee, that something means haranguing an administration witness in an effort to get a point across instead of just asking a straight question "to which you'll get an empty answer."

It can also bring, as it did recently, an explosive reaction from chairman Pete V. Domenici (R-N.M.) who testily accused Riegle of "badgering" Treasury Secretary Donald T. Regan Jr. over the honesty of the administration's midyear economic forecast.

"He didn't look very badgered at the time," Riegle recalled later. In any case, he said, a senator gets only five minutes to ask questions at such sessions and "if you've got a chairman that's on the side of the witness," it isn't easy to get your points across.

"If you are trying to change a policy around here . . . and if you think it's wrong and you're going to challenge it," Riegle said, "sparks are going to fly."

To some extent, Riegle is still dogged by his days in the House where he arrived in 1967 as a brash, 28-year-old freshman Republican. He set out to fight the war in Vietnam and before long let it be known that he intended one day to become President.

He wrote a diary-style book called "O Congress" whose observations ("Bulls--t is an occupational hazard around this place") left ripples some still remember. Increasingly frustrated by the Nixon administration's policies, in 1973 he switched parties and became a Democrat.

An indefatigable campaigner who will stop on a dime close to midnight just to shake hands at a Dairy Queen, Riegle beat out two better-known names for the Democratic Senate nomination in 1976 and then was elected in a bitter campaign against Republican Marvin Esch. Some think Riegle actually benefited from a "backlash" over newspaper revelations that he had had an affair with an unpaid staff member in 1969 while he was still married to his first wife.

As a senator, he has grown more and more parochial and, according to colleagues in the Michigan delegation, been all the more successful for it.

"He was kind of anti-establishment and it irritated a lot of people over here. But most of them are gone now," said Rep. William D. Ford (D-Mich.). "And that was House member Riegle, not Senator Riegle. . . . Now he's settled down to work and become the kind of person he's going to be . . . very pragmatic . . . interested in the issues that are important to Michigan which is a very sick industrial state." Indeed, it may be just as well, in Ford's view, that Riegle isn't preoccupied with cosmic issues of war and peace.

"If Don was painting great swathes through the field of international relations, I don't know if the unemployed auto workers of Michigan would be so pleased," Ford said. "Ralph Nader tagged him the senator from GM. Well, maybe that makes for a laugh at the cocktail parties around town. But it's the greatest boost you could give in Michigan."

A standard Riegle speech will focus on the ills of the economy with a relentless succession of statistics: continuous double-digit unemployment for more than 30 months, the highest jobless rate in the country for more than two years, welfare rolls rising, unemployment benefits running out, and all the while, Reagan's budget cuts callously making "the most violent economic storm since the 1930s" even worse.

"Maybe he sees himself as a point man," suggests Sen. Richard G. Lugar (R-Ind.). "On appropriations bills, he keeps trying to get Republicans on record as opposed to this or that. But the resolutions he offers have a kind of clobbering-your-mother quality to them, such as 'Resolved: President Reagan has failed miserably on Social Security.' He seems to enjoy it. But when you see it coming on, the floor of the Senate empties. He simply tears the skin off everyone in sight."

For all that, Riegle is also widely rated as hard-working and knowledgeable about economic issues, a Harvard Business School Ph.D. candidate who does his homework on the problems that preoccupy him. He emphasizes service to constituents and has about 28 people, half his staff, scattered around the state in seven regional offices. In Washington, his office copes with a deluge of some 600,000 letters a year, three times the volume it was used to before Reagan's election, according to administrative aide Jim Arbury.

One result is that Michigan is now No. 2 in the nation in Urban Development Action Grants, a program that the Reagan administration had put on its "hit list" and one that Riegle has helped keep intact.

"We have 4.1 percent of the population and we get 8.6 percent of the UDAG awards," says Mike Smith, a community development specialist for Riegle in Detroit. "Since 1978, a total of $190 million has gone to Michigan. New York is the only one ahead of us."

Even so, Michigan, while first in the unemployment rate with 14.7 percent (unadjusted) in July, still ranks close to last in the return of the tax dollars it sends to Washington.

The joblessness in some Michigan cities is staggering: 27.7 percent in Pontiac in June (the most recent month for which breakdowns are available); 20.8 percent in Flint, Riegle's hometown; 18 percent in Waterford Township; l7.8 percent in Roseville, a Detroit suburb; 17.6 percent in Saginaw; 17.1 percent in Detroit. Among the 20 hardest hit metropolitan areas in the country, Michigan has seven.

In short, in Riegle's view, he doesn't have the luxury to spend his time on broad issues of foreign policy. Instead he finds himself sending a letter to a small-town zoning board on behalf of a new GM Pontiac plant that the board wasn't sure it wanted.

"That's a pretty fine focus on a Senate job," Riegle acknowledged. "I mean, normally, senators don't do that. . . . But when I see my state being torn to pieces, my impulse is to try to get hold of it and to help everywhere I can."

"There's nothing wrong with what he's saying," said a top aide to another Senate Democrat. "But you could argue that he turns people off. There's too much of it. What everybody says is he's on top of every issue, he throws in bills and amendments on every topic and does not follow through."

There are also some who think his reputation as "a fighter for all of us" is a bit overblown.

"His strategy basically is to champion the auto companies' proposals and to try to get the UAW and the auto companies to be on the same side of all issues," Ralph Nader charged. "His spiritual office is the Fisher Building in downtown Detroit. I have yet to see him take an issue against General Motors, whether it's pollution control, auto safety or anything like that."

Still only 44 and a Senate freshman despite his 16 years in Congress, Riegle feels the fact that he replaced the late senator Philip A. Hart (D-Mich.), the "conscience of the Senate," is the real reason for the disappointment expressed by Nader and others. Hart urged GM's breakup under the antitrust laws and, as Riegle put it, "took a very hard line, as best I recall, on the issues that were facing the auto industry."

Riegle prefers to talk in terms of cooperation between business, labor and government to meet the international competition. "The old adversary system really doesn't work any more," he contended. "We've moved into a new era of economic realities and pressures. And this isn't fully understood yet."

The effectiveness of Riegle's style remains a question mark. Sometimes, his persistence pays off, as in his missionary work on the Chrysler bailout and again last year in restoring the minimum benefit under Social Security.

"He's entitled to a lot of credit he never got on that the minimum benefit ," said Levin. "I think he raised it three times on the Senate floor, he kept forcing the Republicans to stand up and have their votes counted. Finally on the fourth time around, the opposition just caved in. It was somebody else's amendment that passed, but Don raised the visibility of the issue."

Others say it's a different story when Riegle's name is the main one attached to a measure. "Don Riegle's sponsorship of a measure will cost it 10 votes on the Senate floor," asserts one Senate Republican strategist. A Democratic senator who asked not to be named differed only on the number: "I wouldn't say 10, but some."

"When you hear that it's Riegle's amendment, I vote no," a Midwest Republican said. "You don't look at the amendment, you look at the name."

But "you can say that about a lot of liberal Democrats," cautioned a longtime Senate insider. With Riegle, this source suggested, "there are two reasons -- first because he's a liberal and second because he is still a turncoat in the minds of many conservative Republicans."

Added a southern Democrat: "Anyone who is as dogged and passionate in opposing an incumbent administration is going to be acrimonious at times. but there is very much a proper role for an administration critic up here. Maybe Riegle doesn't pass a lot of things, but he doesn't attempt to pass a lot of things. Look at Jesse Helms. He doesn't pass very much either. But he sure helps shape the agenda up here."

Riegle conceded that "in terms of being strident, in terms of pounding something home over and over, on the Banking Committee or the Budget Committee, I clearly have done that. I've done it consciously and knowingly and I will continue to do it because I don't think the country yet has the sense of urgency on economic issues that it needs to have."

Riegle ruffled feathers in Michigan last year when he voted for President Reagan's mammoth tax cut package while Levin voted against it. Riegle said he wanted to give the administration's approach a chance, but the expressions of dismay back home, from the UAW and elsewhere, were instantaneous.

State Democratic Party vice chairman Clyde Cleveland, a Detroit City Councilman, declared angrily that "if you're in office and you sacrifice your principles to stay there, you're a whore."

Don Riegle did not have to be warned twice. "I think the heat he got on that has completely turned him around," Cleveland said. "He's not voted wrong on a thing since then. I think he shocked all of us with that vote . . . but he wasn't the only one. The Democrats kind of followed Reagan like a herd of sheep on that one."

For the upcoming election, Riegle has been thumping hard on the fact that in these times of instant seniority, he is in line to become chairman of the Senate Banking Committee whenever the Democrats regain control of the Senate. He was fourth from the bottom at the start of the year but then the ranking Democrat, Harrison A. Williams Jr., resigned because of Abscam. The next in line, Sen. William Proxmire (D-Wis.) was already the ranking Democrat on the Appropriations Committee and Cranston, the minority whip, decided he was too busy. Williams' resignation also made Riegle the ranking Democrat on the Senate Labor subcommittee.

Riegle wasted no time this spring in announcing to the Detroit News what was about to happen. The resulting column was entitled: "Riegle Tooteth His Horn."

Riegle plans to spend some $2 million tooting it to a second term in his campaign against former congressman Phil Ruppe, the Republican candidate. The money, Riegle says, simply "buys you the same campaign that cost $800,000 in 1976."