In the 15 months through last March 31, political action committees contributed $25 million to 1990 Senate candidates. The most generous PACs represented the energy industry and friends of Israel.

A computer-aided analysis demonstrates the pattern of donations in this well-established sector of U.S. politics. It shows, for example, that PACs consistently give the most to incumbents who serve on the committees with jurisdiction over their interests.

The computer also makes it easier to see how individual senators and challengers become the favorites of certain interest groups. As always in the murky realm of political money, it is nearly impossible to determine whether the PACs targeted the candidates, or vice versa. PAC executives give different accounts when asked who targets whom.

The case of Sen. J. Bennett Johnston (D-La.) is illustrative. Johnston collected $236,500 from oil, gas, utility and other energy industry PACs during this 15-month period, more than any other senator received from any interest group.

At least $35,000 of that amount was donated to Johnston at a fund-raiser last January organized by electric company executives who were attending a meeting in Phoenix of the Edison Electric Institute, a trade association. Walker F. Nolan, executive vice president for governmental affairs for the E.E.I., said the executives decided to throw the fund-raiser the night before Johnston was due to make a speech to the group -- for a $2,000 honorarium.

"He's chairman of the Energy Committee," explained Nolan. "That's why you hold fund-raisers and have PACs, not because we expect any quid pro quo or immediate thing. But if you're going to do fund-raisers, you're doing it for people who have jurisdiction over your issues."

The fund-raiser produced a total of $43,000, including individual contributions, for Johnston's campaign, nearly all of it from PACs affiliated with the electric companies represented at the E.E.I. meeting.

Steven F. Stockmeyer, executive vice president of the National Association of Business PACs, has a different view of who targets whom. He said most PAC money is given because it is solicited by members and their staffs who have fund-raising "down to a science." Negative publicity gives the public the false "image of PACs stuffing money in candidates' pockets," he said. "It just ain't that way. PACs are basically responsive. . . . PACs don't say 'I wish senator so and so would have a fund-raiser. They get dozens of invitations a week and decide which ones they just have to respond to."

The role of PACs in the election process is a critical issue as Senate negotiators try to reach some compromise on campaign finance reform. Leaders of both parties have introduced bills that would ban PAC donations, but the bills are different on many points, and there is no sign yet of a compromise.

While the special interest group PAC money is easier to keep track of, a candidate often receives individual donations from the same pool of executives who donate to PACs. For instance, Johnston received about 45 percent of his funds from PACs in the 15 months through March 31. But an examination of some of his fund-raising events shows how the sources of energy money overlap.

In Houston last September, for instance, a group of oil and gas executives sponsored a fund-raiser for Johnston that raised about $30,000 in PAC and individual donations, according to Bob Mann, Johnston's campaign spokesman.

Also last fall, the chairman of Atlantic Richfield oil company, who Johnston knew from Louisiana, joined with Lew Wasserman, head of the MCA entertainment conglomerate, to host a fund-raiser for the senator at Chasen's restaurant in Los Angeles, Mann added. It raised about $50,000 for Johnston.

Pro-Israel money also flows from PACs and individuals. Pro-Israel PACs gave $1.1 million to Senate candidates through March 31, the fifth highest among all interest groups identified by a Common Cause classification system. Pro-Israel PACs traditionally have focused their attention on key Senate races, giving much less to House candidates. Their average donation is nearly double that of other PAC groups.

The law limits an individual PAC to a maximum contribution of $10,000 to a single candidate -- $5,000 for a primary contest and another $5,000 for the general election. Pro-Israel PACs have given maximum $10,000 contributions 27 times. This is more than double the "maxed out" giving of any other group of PACs. The Desert Caucus, a pro-Israel PAC in Tucson, Ariz., made eight maximum donations in the period. Only the Association of Trial Lawyers of America PAC has given as many $10,000 donations.

Sen Carl M. Levin (D-Mich.) is the leading recipient of pro-Israel PAC money during this period. He got $130,800, including six contributions of $10,000. Gordon Kerr, Levin's campaign manager, said the senator "is a long-standing believer in the U.S.-Israel strategic relationship, and he's a Jewish senator, for gosh sakes."

The Arab-American Institute has criticized Levin and other favorite pro-Israel PAC recipients for doing "favors for the Israel lobby" and "not operating in the interests of the people who elected him." In a recent fund-raising letter targeted at pro-Israel donors, Levin called this a slur. In the same letter, Levin noted he had drafted a letter to the president and "secured 73 Senate signers" opposing a proposed cutback in the $3 billion annual foreign aid package to Israel.

Only two of the top 10 Senate PAC recipients during the first 15 months of this election cycle are Republicans. But three GOP candidates for the Senate -- Reps. Hank Brown of Colorado, Thomas J. Tauke of Iowa and Lynn Martin of Illinois -- have received $500,000 or more from PACs. Of course, all of them share the advantage of being incumbents -- in the House, not the Senate.

Tauke received $95,473 from communications industry PACs, more than any incumbent senator received from that industry. He is a member of the key telecommunications and finance subcommittee of the House Energy and Commerce Committee. Brown was the leading recipient of insurance industry PAC money, with $77,332. He is a member of the tax-writing House Ways and Means Committee. Martin's biggest chunk of PAC money -- $50,750 -- also came from the insurance industry.

John D. Rockefeller IV (D-W.Va.), who spent $12 million -- much of it his own money -- when he was elected in 1984, was the top recipient in total PAC dollars for the 15-month period. His $886,000 in PAC funds was about 43 percent of his total receipts.

Lane Bailey, Rockefeller's top aide, said PAC money "is easier to raise" because "that's their purpose" -- to donate to candidates. "It is harder to pursue individuals and say, 'I need your money.' And for a Rockefeller, that's not an easy task."

Sen. J. James Exon (D-Neb.), a member of the Commerce Committee, got the highest percentage of his money from PACs, more than 60 percent. His chief of staff, Gregory Pallas, said Exon is one of the few senators who only raise money in the two years before an election. Exon seeks out PAC money "out of necessity" because Nebraska is a small state with several other hotly contested elections this year, including a governor's race. In addition, Exon's opponent, former Rep. Hal Daub -- who isn't accepting PAC money -- has been aided by fund-raising appearances by President Bush.