Sen. Bob Packwood (R-Ore.) had a problem. For years the Pacific Livestock Exhibition in Portland - a famous annual event - had been financed by the parimutuel profits from 10 days of horseracing during the exhibition. Then, unexpectedly, the Internal Revenue Service demanded that taxes be paid on these profits - all the way back to 1959.

Luckily for Sen. Packwood, he also had a friend - his colleague Russell Long, a Democrat from Louisiana but also Packwood's chairman on the Senate Finance Committee.

Packwood took his problem to his friend, and before long the United State Internal Revenue Code contained a new amendment, one explicitly exempting from taxation earnings from horseracing meetings like those held by the livestock exhibition in Portland.

It has become one of Washington's cliches that Russell Long is a wily, crafty, powerful senator. It is a cliche that is revived almost annually at this time of year, when Long usually finds himself in a crucial position in one or more House-Senate conferences on important legislation.

Like most cliches, this one is rooted in the truth. But the precise nature of Long's power and success is too complex to fix neatly into any cliche. A full examination of the Long phenomenon is an education in the ways of the U.S. Senate.

Gaylor Nelson, a liberal Democratic senator from Wisconsin and a friend of Long's says he is tired of giving interviews on the subject of the chairman of Finance. "It doesn't make news," Nelson complains, to report that Long's success depends on intelligence and diligence instead of mirrors and tricks.

Long has no better booster than Nelson, so this testimony may be somewhat suspect, but a wide variety of senators, Senate aides and other Long-watchers agree that Long's power and influence are not a matter of trickery or guile. Packwood is a good example.

"Most of it is not trickery," Packwood said in an interview. "Russell is the smartest legislator I've ever met . . . I've never seen him unprepared . . . He's absolutely fair, and he'll go to bat for you. He'll make phone calls."

Or write letters, or put a colleague in his favorite hammerlock hold, elbow bent around his head, and whisper persuasively - or do anything be can think of to help himself or a friend or just a temporary ally get what they need from the political game that Long so enjoys playing.

The crucial point about Long, Nelson said, is a simple one: "He's got the troops."

Numerous Senate-watchers agreed; Senators vote with Long not because they are tricked or intimidated, but more often than not because Long's political ear is perfectly attuned to the cacophony of senatorial sonatas. Long likes to sense the mood of the Senate, then act as though he is leading it, according to an aide to a leading Senate liberal.

"He represents a lot of the maintstream political pressures," according to Robert M. Brandon of Ralph Nader's Tax Reform Research Group. In other words, Long is a specialist at placing himself on the winning side. Early.

From more than a dozen interviews, these emerged as the principal sources of Long's enormous Senate power: his position as chairman of Finance, his intelligence, his political skill, his energy and diligence and the mood of the Senate today.

Merely by serving as chairman of Finance, Long enjoys enormous powers. Despite reforms, the Senate still defers to its chairmen, and no committee is more influential than Finance. Most of the legislative initiatives proffered or promised by President Carter - from energy to welfare reform to national health insurance - must pass through Finance.

Long cultivates the committee the way an orchid lover cares for his flowers. When Packwood came to him with his problem, Long was no troubled by questions of party membership or political philosophy. The fact that Packwood belonged to his committee was enough to win his full support.

He is utterly loyal to his friend Nelson, though the Wisconsin liberal once blew up at Long for conducting committee business too secretively, and regularly votes against him on the Senate floor. Nelson is one of three Liberal voices on the committee - the others belong to Floyd K. Haskell (D-Colo.) and William D. Hathaway (D-Maine). Haskell is already engaged in a tough re-election campaign, and Long is busily helping him, attending fund-raisers and apparently encouraging his friends to help Haskell, too.

Recent reports of campaign contributions filed by Haskell show thousands of dollars contributed to him by Louisana oilmen and their lawyers.

Long happily shares power for at least the appearance of power among Finance Committee members to win their loyalty and cooperation. He has given Sen. Spark M. [WORD ILLEGIBLE] (D-Hawaii) chairmanship of a subcommittee on sugar, a crucial subject for Hawaii. He put another freshman, Daniel Patrick Moynihan (D-N.Y.), in charge of welfare matters.

Long also helped Moynihan redeem his No. 1 campaign promise by attaching a rider to the Social Secretary bill that would provide hundreds of millions of dollars in relief for welfare costs to New York State. With disarming candor, Moynihan admits that he gives as many votes as he can to Long in the committee in return for the favors Long has done him.

The two senior Republicans on Finance are intimate Long allies, Carl T. Curtis (Neb.) and Clifford P. Hansen (Wyo.).On recent Sunday mornings Long has been meeting with Republican committee members for "Sunday sermons" in Hansen's apartment to keep the minority informed of what President Carter and his aides are telling Long about the energy legislation.

Sen. Bob Dole (R-Kan.), another Republican member of Finance, reacts somewhat uncomfortably to the suggestion that Republicans on the committee usually vote with Long. Dole said he wasn't sure he was always with Long, because he wasn't sure he always knew where Long stood. In fact, though, the Republicans very often do support the chairman.

Stories of Long's energy and intelligence loom large in Senate mythology: he has prodigious amounts of both. He is said to know the tax code better than any member of Congress since the retirement of Rep. Wilbur Mills (D-Ark.). He gets up draws on committee staffs regarded as the best on Capitol Hill for briefings and memorable. And he takes reading matter home to bed.

Moynihan was pleased with himself the other morning for getting up and arriving ahead of time at a meeting of the energy conference. "But Long was already there," he recalled.

Long devotes at least as much time and thought to Senate politics as he does to the substantive issues before his committee.

"He loves to work the Senate floor," as one aide put it. Long spends hours chatting and joking with colleagues on the floor, and his endless reserve of good humor (and good jokes) has served him well. Some remember a nastier Russell Long from the days in the 1960s when he had a serious drinking problem (which he now acknowledges), but today's Long (who takes on occasional drink) is one fo the most popular and congenial senators.

Avoiding grudges is a cardinal political rule for Long. He often tells associates never to carry over emotions from the last fight into the next one - start fresh every time, and maximize your number of allies every time. Long's skill and power are often compared to Lyndon B. Johnson's or Robert Kerr's, but old hands say he is less vindictive and less vain than either of those Senate titans.

Behind Long's good nature, though, is a discipline and calculating politican, generally thought to be the best counter of votes in the Senate. He carries vote tailies on tiny lists which he can hold in the palm of his hand, and seldom makes a mistake.

"I might be on the floor [managing a bill]," Nelson said, "and think I'm going to win 60-40, and lose it 51-49. Russell would never do that."

And naturally, Long is a master at improving the count in his own favor. He is a consummate horse trader with a fine sense of timing, and knows the value of a favor. Every Long-watcher has a collection of stories about the chairman's traders.

For example, in drafting the tax bill in 1976 Long at first strongly favored a provision eliminating tax deductions for trips taken by Americans to business conventions in foreign countries. But Sen. Daniel K. Inouye (D-Hawaii) fervently opposed this amendment, and went to Long for help when the bill came to the floor.

Using a tactic he employs often. Long asked Inouye to help him pass or defeat a number of amendments he thought would be close, and in return offered to back Inouye's amendment to allow overseas conventions. Inouye waited - and voted as Long asked - through amendment after amendment - until the chairman finally brought the Inouye amendment to the floor. He got it passed.

In another close fight, senators opposing Long were surprised to see Sen. Long A. Durkin (D-N.H.) change his vote at the end of the roll call to line up with Long.

One of them went up to Durkin to find out why.He returned to ereport: "Durkin just got his heating oil tax credit" - an amendment to the energy bill to benefit New Englanders that Durkin had been trying to get through Long and the Finance Committee for a long time. To get that one vote, Long agreed to back the credit.

Long's reputation as a miracle worker who always gets his way is simply wrong. He has lost big fights, and has often compromised or retreated in the face of an apparent defeat. For example, he was a supporter of the idea originally advanced by Vice President Rockefeller to create an energy development trust fund and a state corporation to foster new energy intentions, an idea that never got through his committee.

On a recent and potentially tense confrontation with other Senate committees over the jurisdiction of Finance. Long similarly retreated, allowing the Appropriations Committee to consider a package of tax credits which its members thought amounted to appropriations. (Then he beat the Appropriations Committee on the merits of the case of the Senate floor).

Another crucial aspect of Long's strategy is to avoid taking strong personal positions on potentially unpopular issues.

As Long himself likes to point out, he had originated very few ideas over the years that ended up in legislation. His principal concern has always been the oil and gas industries. He is personally a millionaire thanks to oil royalties inherited from his famous father, Huey, and has staunchly supported the industry that is Louisiana's most important. He has been similarly loyal to his state's shipbuilding industry.

(Liberals over the years have attacked Long for rendering to these "special interests," but this argument doesn't seem to wash in the Senate, where representing constituent interests is an [WORD ILLEGIBLE] practice in almost any circumstance).

Long likes to think of himself as something of a "populist," and he has done a good deal for the nation's poor. He is proud of the earned income credit, a tax rebate for low-income workers, and his support for various welfare programs.

But in a recent Senate debate he stood up to defend tax deductions for businessmen's first-class airline tickets, saying the airlines would abolish first class if the deductions was disallowed.

"I know when I travel I oftentimes go first class because my wife is like my mother used to be," Long told the Senate. "She is always bring along strawberries for someone. To have foot space we need first-class seats with all the baggage Mrs. Long tends to bring aboard.

One personal idea Long has fought hard for is the employee stock owner ship plan he has put into federal law. In 1976 he won congressional approval for a tax credit allowing companies to buy their own stock for their employees at a tax savings.

This one was not too popular among his colleagues, not least because it amounts to federal financing of stock purchases which allow companies to retain their own earnings Long tried an unusual gambit to win support for the idea, writing a letter to the wife of each of his Senate colleagues. These letters - each delivered by messenger to senator's homes - began with this paragraph:

One should not underestimate the power of a wife. If the wives of senators understand an issue it makes a different - for the better."

Perhaps so. The proposal sailed through the Senate when it came up for a vote.

Several Long-watchers said the senator's biggest advantage besides his personal skills is the mood of the Senate, which is receptive to his kind of leadership. The liberals complains, as one Senate aide put it, but they don't have the votes. "Their flaw is that they think their arguments speak for themselves."

A fellow senator said Long is that sort of man who thinks American society is working well, who believes in encouraging private capitalists, who is not distraught by poverty or class struggle. His philosophic acceptance of the status quo, this senator suggested, is right in step with the senatorial mood. Several other Senate sources agreed.

Russell Long is 59 and by all indications is in top form. "When everybody gets tired, he isn't tired." Moynihan observes with evident envy. His dominance in the Senate could continue for years.

In fact, one Senate aide calculated the other day that if Long should live as long as Sen. John L. McClellan (D-Ark.), who died last month, he would be dominating the Senate until 1999.