Corporate political action committees are shaking their money trees, and large sums are falling into the campaign treasuries of congressional candidates.

Over periods of 18 to 20 months, each of 10 Senate candidates has received at least $100,000 from the companies' PACs, and one, Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee, has gotten $258,195. Labor PACs chipped in an additional $20,150 for Long.

Across the Capitol, Long's counterpart, Chairman Al Ullman (D-Ore.) of the House Ways and Means Committee, got $105,413 from corporation units, becoming one of two House candidates to top $100,000. He got $23,600 more from union PACs.

Five House Republicans who are seeking Senate seats received a total of more than $825,000 from company PACs. The pacesetter, Rep. Steve Symms (Idaho), listed receipts totaling $207,040. Union PACs gave him $1,000.

Senators who aren't running received -- to pay off old election debts -- $520,482 from corporate units. The bulk, $406,772, went to 10 Republicans, the rest to six Democrats. The leader, with $130,400 was Sen. Charles H. Percy (Ill.), the only Republican in the group who also got labor money ($18,900).

These findings emerge from a Washington Post study of Federal Election Commission data on 1,222 corporate and 308 union PACs in existence from Jan. 1, 1979, to June 30, 1980, in some cases and to Aug. 31, 1980, in others. A few of the units never became active or were abandoned.

The recipients were 1,016 congressional candidates, including many who lost primary or runoff races. They got $10,704,561 from the corporate units, and $6,571,067 from union units.

Additional large amounts continue to flow into the candidates' coffers. In the two-month period ended Aug. 31, the FEC said, they received $7.4 million -- $119,355 a day -- from PACs of all kinds, including those of trade and professional associations and of conservative groups.

Realistically, the study's finding of a swiftly widening dollar advantage of corporate over labor PACs is a substantial understatement, because many candidates getting significant sums from corporate PACs are, at the same time, favorites of the association and well-heeled conservative PACs, not to mention numerous corporate owners and officers and their spouses.

At the same time, unions draw on members for unreported, "soft money" help to favored candidates, such as staffing telephone banks and ringing doorbells.

Corporate PACs made 25 contributions in the $50,000-$100,000 range, for a total of $1.6 million, compared with union PACs' 22, for a total of $1.5 million. Other ranges: $40,000 to $50,000 -- corporate 17 ($750,356), labor 8 ($358,609); $30,000 to $40,000 -- corporate 50 ($1,002,552), labor 35 ($1,108,807); $25,000 to $30,000 -- corporate 30 ($821,714), labor 22 ($575,687); $20,000 to $25,000 -- corporate 39 ($860,306), labor 29 ($642,855); $15,000 to $20,000 -- corporate 62 ($1,031,075), labor 42 ($717,171); $10,000 to $15,000 -- corporate 89 ($1,119,969), labor 56 ($715,686).

Of the $8.9 million contributed by corporate PACs in the $10,000-and-up categories, Democratic incumbents got 41 percent and Democratic challengers 1 percent, while GOP incumbents got 34 percent and GOP challengers 22 percent. A modest sum went to candidates in open races.

Of the $6.4 million given by union PACs in the same categories, Democratic incumbents got 78 percent and Democratic challengers 13 percent, while GOP incumbents got 4 percent and GOP challengers 0.5 percent. Candidates for vacant seats, all Democrats, got 3.5 percent.

A mere 37 candidates -- those in the $50,000-and-up bracket -- got nearly one-third of the corporate PAC's $8.9 million, while a mere 28 got slightly more than one-third of the union PACs' $6.4 million.

Of all corporate PAC congressional contributions, regardless of amount, 823 House candidates got $7,043,507, or two-thirds, while 193 Senate candidates got $3,661,054. The figures for the labor PACs: House, $4,176,818, or 63 percent, and Senate, $2,394,249, or 37 percent.

The growth in the number of corporate PACs has been explosive. At the end of 1974 there were 89, mainly because for three-quarters of a century it was a crime for a corporation to spend company funds for federal candidates. Unions, under a similar prohibition, had 201 units.

A drastic change in the situation occurred Nov. 24, 1975, when a divided FEC voted to permit corporations to spend company money to set up and operate PACs empowered to solicit their stockholders (the equivalent of union members), directors and officers and mid-level and professional employes. uAt the time, 139 corporate and 226 labor PACs were registered.

Stimulated by the ruling, the number of corporate PACs more than doubled, to 294, in less than six months, and more than tripled, to 433, in 13 months. By the end of 1977 there were 550, by the end of 1978, 784, and by the end of 1979, 919. Last July 1, when the latest count was made, the total was 1,106. Meanwhile, union PACs increased by 29, to 255.

Moreover, the potential for increased numbers of PACs is widely recognized to be far higher among corporations than unions. "Corporate PACs have only begun to exploit the opportunity provided by current legislation," said John G. Murphy, a Georgetown University law professor who was the FEC's first general counsel.

The growing clout of corporate PACs also emerged in the presidential campaigns in the period before President Carter and Ronald Reagan won their parties' nominations and precluded further private financing by accepting $29.4 million of public funds.

According to The Post's analysis of the FEC data, corporate PACs gave $1,024,647 to presidential aspirants, while union PACs gave $197,395. Although the corporate units' consolidated contributions to Carter $261,866, exceeded those to Reagan by $19,728, they gave an additional $400,578 to three of his defeated rivals for the GOP nomination: George Bush, now Reagan's running mate, John B. Connally and Sen. Howard H. Baker Jr. (R-Tenn.).

Labor PACs gave Sen. Edward M. Kennedy (D-Mass.) $90,463 and Carter $80,642, while contributing nothing to Reagan and a total of $2,400 to Bush, Connally and Baker.

Many corporate and labor PACs are not antagonists, and may even reinforce each other, partly because they often make common cause in, say, seeking government help for a particualr industry, and partly because they take pragmatic approaches.

A case in point is the PAC of Winn-Dixie Stores, the fifth-largest grocery chain. While targeted for a lengthy boycott by organized labor because of its antiunion stance, it contributed large sums not only to sympathetic legislators but to legislators heavily dependent on union PACs.

In the current campaign, however, five chemical companies are under attack by Environmental Action, a non-profit group. It calls the firms the "Filthy Five" because of "their long histories of pollution violations coupled with very heavy levels of campaign spending," and has gotten pledges from 63 House and Senate candidates to refuse money from the firms' PACs.

The companies -- Dow Chemical, International Paper, Occidental Petroleum, Republic Steel and Standard Oil of Indiana (Amoco) -- had given a combined total of more than $630,000 to federal candidates by the end of last month. Environmental Action identified Dow Chemical as the top giver, saying it has eight separate PACs which together have contributed $206,750.

Listed after Dow were Amoco, $145,800; International Paper, $119,049; Occidental (including substantial giving by its chairman, Armand Hammer), and Republic, $76,125.

Additional findings from The Post's analysis:

Senate candidates getting at least $100,000 each from corporate PACs: Long, $258,195; Rep. James Abdnor (R-S.D.), $187,350; Rep. Dan Quayle (R.-Ind.), $162,061; Rep. Charles E. Grassley (R-Iowa), $159,215; Rep. Steve Symms (R.-Idaho), $145,975; Percy, $130,400 (to pay old campaign debts); Sen. Bob Dole (R-Kan.), $116,995; Sen. Herman E. Talmadge (D-Ga.), $114,370; Sen. Ernest F. Hollings (D-S.C.), $107,625, and Sen. Warren G. Magnuson (D-Wash.), $106,850.

Senate candidates getting at least $100,000 each from labor PACs: Sens. Birch Bayh (D-Ind.), $164,375; John C. Culver (D-Iowa), $127,520; Magnuson, $115,921; George McGovern (D-S.D.) $111,372; Gaylord Nelson (D-Wis.), $102,870, and Alan Cranston (C-Calif.), $100,500.

House candidates who got at least $100,000 each, all from corporate PACs:

Rep. John E. Porter, (R-Ill.), $105,925, and Rep. Al Ullman (D-Ore.), $105,413.

Senate candidates getting between $50,000 and $100,000 each from corporate PACs: Cranton, $94,032; Sen. Richard Stone (D-Fla.), $74,995; Sen. Barry M. Goldwater (R-Ariz.), $74,195; Sen. Mike Gravel (D-Alaska), $73,963; Sen. John Glenn (D-Ohio), $73,193; Sen. Bob Packwood (R-Ore.), $72,390; Sen. Jake Garn (R-Utah), $72,164; David C. O'Neal (R), $68,582; Sen. Daniel K. Inouye (D-Hawaii), $64,467; Sen. Wendell H. Ford (D-Ky.), $59,758; Sen. Daniel P. Moynihan (D-N.Y.), $58,300 (to pay odd debts); Sen. Donald Stewart (D-Ala.), $54,672; Sen. Robert Morgan (D-N.C.), $53,546, and Nelson, $52,475.

Senate candidates getting $50,000 to $100,000 each from labor PACs: Sen. Thomas F. Eagleton (D-Mo.), $99,850; Sen. Charles McC. Mathias Jr. (R-Md.), $95,050; Stewart, $93,918; Gravel, $91,800; Glenn, $86,300; Alan J. Dixon (D-Ill.), $86,250; Sen. John J. Durkin (D-N.H.), $73,500; Sen. Jacob K. Javits (R-N.Y.), $71,800; Sen. Frank Church (D-Idaho), $69,150; Rep. Christopher J. Dodd (D-Conn.), $67,090; Sen. Patrick J. Leahy (D-Colo.), $62,544; Zell Miller (D-Ga.), $54,808; Richard A. Pettigrew (D-Fla.), $53,250.

House candidates getting $50,000 to $100,000 each from corporte PACs: William H. Royer (D-Calif.), $95,370; Rep. James D. Santini (D-Nev.), $69,549; Majority Leader Jim Wright (D-Tex.), $69,541; Rep. Steve Symms (R-Idaho), $61,250; Thomas W. Pauken (R-Tex.), $58,221; Rep. Phil Gramm (D-Tex.), $56,869; Jack M. Fields (R-Tex.), $56,800; Margaret Roukema (R-N.J.), $51,214; Rep. Richard C. Shelby (D-Ala.), $50,225, and Rep. Guy Vander Jagt (R-Mich.), $50,197.

House candidates getting $50,000 to $100,000 each from union PACs: Rep. James C. Corman (D-Calif.), $75,108; Gary R. Goyke (D-Wis.), $63,900; Rep. Frank Thompson Jr. (D-N.J.), $60,200; Rep. John Brademas (D-Ind.), $55,100; Carey E. Peck (D-Calif.), $54,870; Rep. Michael D. Barnes, (D-Md.), $54,721; Rep. John M. Murphy, (D-N.Y.), $52,200, and Rep. John Burton (D-Calif.), $51,400.