WASHINGTON -- After 24 years on Capitol Hill, Sen. Paul S. Sarbanes is finally poised to claim real power. If he's re-elected this fall, and Democrats keep control of the Senate, he'll become chairman of an important committee for the first time.

But the source of his potential clout -- his longevity in office -- is paradoxically his biggest weakness. Anti-incumbent fever is running high, and Mr. Sarbanes has few high-profile accomplishments to show for his long tenure in Washington. Content to remain in the shadows, cautious in the extreme, he isn't regarded as a leader on any major issue.

Statewide polls show weak enthusiasm for keeping him in Washington through the year 2000. (Only 34 percent of Maryland voters said they were committed to his re-election in a survey taken this year.)

And yet, Mr. Sarbanes has managed to win handily in the past despite weak pre-election numbers and is favored to win again. The 61-year-old senator faces no serious primary opposition, and Maryland is among the most heavily Democratic states.

In a recent speech, Mr. Sarbanes played up his long years in office.

"When I first came here to Congress I was very critical of the seniority system," he said. "But I have to confess to you, as the years have gone by I have come to see more and more virtues in the working of our system."

The chief "virtue" of the system is that a senator can become chairmanof an important committee simply by sticking around long enough and floating to the top. That's just what could happen next year to Mr. Sarbanes. With Senate Banking Committee Chairman Donald W. Riegle retiring, the Marylander is in line to replace him in January.

Republicans who are fighting for the chance to oppose Mr. Sarbanes this fall already are using his longevity in office against him. At least two of the challengers, C. Ronald Franks and Ruthann Aron, label him a "career politician" who should be retired.

That Mr. Sarbanes may be vulnerable to such attacks says something about the mood of the electorate this year -- and perhaps even more about Mr. Sarbanes. Despite a lifetime in public life, this son of Greek immigrants who worked in his father's Salisbury restaurant, then distinguished himself at Princeton, Oxford and Harvard Law, remains an enigma to many Marylanders.

Within the Senate, where he has served since 1977, he has been eclipsed by many of those who were elected with him or later, including George Mitchell of Maine, the retiring majority leader; Daniel Patrick Moynihan of New York; Bob Kerrey of Nebraska; Bill Bradley of New Jersey; and his high-octane Maryland colleague, Barbara A. Mikulski.

While compiling an unambiguously liberal voting record, he has shied away from leadership on high-visibility subjects. He rarely draws public attention to his work in an aggressive way and introduces relatively few bills.

Instead, he concentrates on obscure national and international questions, and does what he can for his political base: organized labor, Greek-Americans around the country and the folks in Maryland.

His defenders say he is cerebral and analytical, able to wield backstage influence through his powers of persuasion and careful not to take a stand until he weighs all sides.

But for years he's been burdened by the label of "stealth senator," and with voters increasingly demanding action in Washington, Mr. Sarbanes will find himself pressed as never before to defend his record.

Former Sen. William E. Brock, another of the Republican Senate hopefuls, says, "It's real hard to find any evidence that he's been there."

Adds Brad Coker, president of Mason-Dixon Political Media Research, a Columbia polling firm, "He is very susceptible to the charge that he gets elected and goes to Washington and we never hear from him for five years. . . . That's been his pattern for three terms."

Mr. Sarbanes, who is expected to formally announce his candidacy for re-election this month, began shifting into campaign mode several months ago, stepping up his fund-raising activities, making more appearances around the state and beefing up his Senate staff with a new public relations person.

But he is still honing his campaign message.

In a recent interview, he gave a rambling response when asked what he thought his legacy would be if he left the Senate now.

"I think we've given the people of the state and the people of the country, since you asked on national issues, you know, representation of integrity and intelligence and independence of judgment and, you know, I think we've done each of the responsibilities that sort of devolves upon us -- because a lot of the responsibilities you get depend on the committees on which you sit or the subcommittees which you chair or so forth and so on -- and I think in each of those instances, we've met those responsibilities and I would think met them to a high standard.

"I guess that's the thing people have to judge. So we've done the job. I mean we've been working at it and, I think, done it well and I think we've not given people any reason to lose confidence in their government. I think if anything we've given them reason to have more confidence in their government."

Mr. Sarbanes' caution is legendary. Ask him what he thinks on an issue, and often he will recite facts, studiously avoiding a hint of his position. Even when he has a position, he seems reluctant to disclose it.

Interviewed in January about the Clinton health reform plan, he avoided taking a specific position -- as if he hadn't had time to digest a plan whose details had been widely publicized and analyzed for months. By contrast, Ms. Mikulski told a reporter that she had serious concerns about the plan's impact on businesses and its reliance on government regulation.

Former Maryland Attorney General Stephen H. Sachs, a longtime Sarbanes admirer, has a theory about the Sarbanes caution. He says that former Sen. Joseph D. Tydings, a liberal, took public positions on controversial topics and ultimately paid, losing his seat in 1970.

Taking a lesson from that experience, "Sarbanes has not focused on issues that pick at the scabs," Mr. Sachs noted.

Other admirers say Mr. Sarbanes simply has an aversion to publicity -- not a lack of influence. While he has never been part of the elected Senate leadership, his friend Mr. Mitchell, the majority leader, calls Mr. Sarbanes "one of the smartest persons I've ever met" and says he relies on him "a great deal for advice and counsel."

But when asked to name issues on which the Marylander helped turn other senators around, he could cite only one: the 1991 extension of unemployment benefits.

Mr. Sarbanes explains his low profile this way: "If you share credit or indeed let others take credit, often you can get a lot of things done. I don't mind doing that. In fact, I think it helps us probably to get the substance done.

"Then I end up having, every time a re-election comes around, having this problem, 'Well, you know, you haven't been blowing your horn, so what have you been doing?' Then the record ought to be subject to some analysis. 'Did he really do anything or have we not been doing anything?' I think we've really done a lot of things."

Asked about his accomplishments, he reels off a list of measures that brought jobs and money to Maryland, including the Chesapeake Bay cleanup, the Washington Metro subway system, Baltimore's light rail line, highways and military facilities. He played a key role in writing a comprehensive 1991 transportation law that, among other things, authorized $160 million for construction and equipment for Maryland's commuter rail service and also changed a formula to more than double, to $9 million a year, money that the system gets for modernization.

In 1987, he conducted the only filibuster of his Senate career in an effort to prevent the transfer of Washington's airports, Dulles and National, to a regional authority dominated by Virginia -- a move that Maryland interests feared would harm Baltimore-Washington International Airport. The filibuster was shut off on the second attempt, but only after an intensive lobbying effort and a promise that BWI would get added federal funds.

As if to emphasize his low-profile nature, Mr. Sarbanes cites as two of his achievements passage of a "demonstration act" designed to test new approaches to ending homelessness and financing public housing, and another measure that makes it easier for the government to dispose of troubled public housing properties.

Backed solidly by organized labor, Mr. Sarbanes is a tireless defender of the interests of federal workers. That's a political imperative for a senator with 275,000 of them as constituents, a sizable bloc in a state where 600,000 votes could be enough to win the Senate race. This year, Mr. Sarbanes worked to end a Republican filibuster that was blocking action on legislation that allows the federal government to offer employees $25,000 buyouts.

Mr. Sarbanes, who gets a substantial amount of his campaign contributions from Greek-Americans, has also been aggressive in protecting Greece's interests in the Senate. Among other things, he helps assure that Greece gets $7 in foreign aid for every $10 that Turkey gets, according to a Senate aide.

Calling the 7-10 ratio a stabilizing influence that has helped to maintain a military balance, Mr. Sarbanes defended his advocacy of Greek interests by saying, "I'm the first Greek-American ever elected to the Senate. The community, I think, takes pride in that. I've always cherished my heritage and the values that my parents gave to me that flowed out of that heritage, which is very much at the center of what American democracy is about, as a matter of fact."

He is a fiercely partisan Democrat. An analysis of Senate voting records last year by Congressional Quarterly found that only one other senator supported President Clinton as often as Mr. Sarbanes did -- Senator Mitchell, whose job is pushing the president's program. (Mr. Clinton returned the compliment recently by mentioning Mr. Sarbanes as a possible Supreme Court nominee.)

During the Bush administration, Mr. Sarbanes was the Senate's most outspoken critic of ambassadors who were appointed solely because they were big contributors to the Republican Party. But with a Democrat in office, he has muted his criticism. He has voted against only one Clinton appointee, M. Larry Lawrence, a wealthy Californian and major Democratic donor who was named ambassador to Switzerland.

Only on rare occasions has Mr. Sarbanes had a leading role in controversial matters. As a member of the House Judiciary Committee in 1974, he wrote one of the key articles of impeachment against President Richard M. Nixon. As a relatively new senator, he led the Senate fight for approval of the Panama Canal treaties in 1978.

Mr. Sarbanes may get another high-visibility role, one that he might well prefer to avoid during an election season, as the Senate moves toward hearings on the Whitewater affair by the Banking Committee.

It is as chairman of that committee next year, say his supporters, that Mr. Sarbanes will finally have an opportunity to shine. One of the Senate's most powerful committees, the Banking, Housing and Urban Affairs panel plays a pivotal role in the financial services and securities industries, as well as in public housing and mass transit.

But he has never been a key player on the committee, having exhibited little interest in domestic banking issues, according to lobbyists and consultants who closely follow the committee. He has preferred to concentrate on housing issues (he heads the housing subcommittee), international monetary questions and the structure of the Federal Reserve. His most visible role has been as a frequent, partisan critic of the Fed for raising interest rates.

"He is something of a cipher," said Karen D. Shaw, a Washington banking consultant, of Mr. Sarbanes' role on the Banking Committee.

Says one lobbyist, who asked not to be named, "His public reputation is consistent with his 'inside' reputation. Insiders think of him as very cautious. They don't think he's a guy other senators look to for a leadership role. He's not a swing vote on any issue."

On-the-record assessments are kinder. Ed Yingling, director of government relations for the American Bankers Association, says Sarbanes' seniority frequently puts him on conference committees where the final form of legislation is shaped. "In conference, he's very active, he's very knowledgeable and he's the strongest debater when they go around the table." But he could not recall any cases "on the banking side" where Mr. Sarbanes turned an issue around or shaped the outcome.

If Mr. Sarbanes has an agenda for the committee under his stewardship, he refuses to say what it is, and some Sarbanes watchers believe he might serve in the job for only two years.

They note that he prefers his work on the Foreign Relations Committee and that, given the choice, would like to assume the chairmanship of that committee if Sen. Claiborne Pell, 75, the current Foreign Relations chairman, decides to retire in 1996.