In the congressional races that will determine the partisan and ideological makeup of the 98th Congress, a key segment of the business community has become a de facto arm of the Republican Party, providing targeted money in key marginal contests for House and Senate seats.

Working in tandem with conservative political groups, this coalition of companies, wealthy oil producers and investors, and business organizations is leading a steady transformation of election financing, disclosed in campaign spending reports at the Federal Election Commission.

The money from this coalition is producing crucial early support and credibility for Republican challengers confronting potentially vulnerable Democratic office holders in the House and Senate.

It is also shoring up the defenses of Republican incumbents -- particularly the GOP members of Congress elected in 1980 who likely would be most vulnerable to any voter backlash this fall against Republicans because of economic troubles. While this support is not the only factor in some 45 key House races and six to eight pivotal Senate contests, it could determine outcomes in the closest of these targeted races.

Timing is one element of the transformation. The heavy flow of support in the earliest stages of campaigns for unknown challengers or junior House and Senate members is a sharp break with pre-1980 custom, when contributions generally favored veteran incumbents. The other is the one-sided concentration of support from this coalition for Republicans.

The business and special-interest groups providing this backing to the GOP fall into four categories: broad-based business organizations, including the U.S. Chamber of Commerce and the National Federation of Independent Business; such major trade associations as the National Association of Realtors and the Associated General Contractors; a relatively small number of corporate political action committees (PACs), many with headquarters in, or with major investments in Sun Belt states; and other political action committees financed largely by independent oilmen in Texas, Louisiana, Oklahoma, Colorado and Southern California, whose primary goal is to elect conservative candidates in the northern tier of states.

Business community support has shifted over the past four years toward Republican candidates, but these groups are distinctive in a number of respects: they are willing to get into races early, often during primaries; many are willing to contribute large amounts, often up to the maximum of $5,000 per candidate; and, unlike many political action committees seeking to maintain access to incumbents, these organizations tend to target the marginal races, often financing challengers or those with neither seniority nor firm grips on their districts.

They share a common goal with major conservative groups such as the National Conservative Political Action Committee, the Fund for the Survival of a Free Congress, Citizens for the Republic and the National Congressional Committee: to shift Congress to the ideological right.

In the overwhelming majority of contests this fall, this has meant financing Republicans' campaigns.

To the extent that these organizations support Democrats, the backing is limited almost entirely to those who backed the administration's 1981 budget and tax cuts over the objections of the House Democratic Leadership. In addition, contributions to Democrats are limited almost entirely to conservative candidates in primary elections in solidly Democratic districts facing opponents who are more liberal.

The groups disavow any commitment to the Republican side. "We are nonpartisan," said Joseph J. Fanelli, president of the Business-Industry Political Action Committee (BIPAC), a comment echoed by spokesmen for the others. But an examination of contributions on file with the Federal Election Commission show the following:

The business organizations. On Aug. 20, the Chamber of Commerce issued a summary of 99 House and Senate races recommended for "business involvement." In only one case -- the Senate candidacy of Rep. James D. Santini (D-Nev.) -- is the endorsed candidate a Democrat, and that endorsement is limited to his challenge in the Democratic primary to Sen. Howard W. Cannon.

BIPAC, which puts what amounts to a business "Good Housekeeping Seal of Approval" on candidates that leads to contributions from other PACs and individuals, has given $112,010 to House and Senate candidates. Of that, $99,230 has gone to Republicans.

BIPAC endorsed nine Democratic candidates and gave them a total of $12,780, but the endorsement was made only in primary elections and was limited to conservative Democrats running against more liberal Democratic opponents. Every candidate endorsed by CIPAC in the general election is a Republican.

Similarly, by the end of June, the National Federation of Independent Business had given Republican candidates $79,650 and Democrats $12,100. All of the money going to Democrats went to Boll Weevil House members who had bucked the Democratic leadership to support the Reagan budget and tax bills of 1981, and to two of the most conservative Democratic senators, Lloyd M. Bentsen (Tex.) and Edward Zorinsky (Neb.)

The Corporate Political Action Committees. Campaign spending records show that some 20 major corporations are particularly active in making early contributions, heavily weighted toward Republicans. The leaders include the Fluor Corp., the Eaton Corp., Dart & Kraft Inc., Standard Oil Co. (Indiana), and Sears, Roebuck and Co.

The Sears PAC this year has given $42,850 to Republicans, most of it to GOP House members holding marginal seats. The list of GOP recipients reads, in many respects, like a list of key races put out by the Republican Congressional Cmpaign Committee.

The PAC has given $12,400 to Democrats, $7,950 of which was to Southern supporters of the Reagan program. Much of the remainder went to Democrats representing districts near Sears' Chicago headquarters.

With even stronger leanings to the GOP, the Eaton Corp. PAC has given $46,150 to Republicans out of a total of $51,650. Fluor Corp.'s PAC has given a total of $85,550 to GOP candidates and committees, and $13,650 to Democrats. In both cases, the Democrats who received money generally were Southern Conservative supporters of the Reagan program.

The trade organizations. Key supports of marginal Republican candidates, often providing up to the $5,000 maximum, have been such groups as the Realtors PAC, the American Medical PAC, the Sheet Metal and Air Conditioning PAC, and the Associated General Contractors PAC. These groups have raised large sums for the 1982 election. Through June, the Realtors PAC has raised $2.2 million and the American Medical PAC $2.17 million.

Through April of this year, the Realtors PAC reported giving $221,176 to Republicans and $112,758 to Democrats. Though on the surface this is a two-to-one split, just over $72,000 of money earmarked for Democrats went to Boll Weevils. This means that the organization gave well over three quarters of its money to participants in the Republican-conservative Democratic coalition that dominated much of the 97th Congress.

More important, the Realtors gave donations of $2,000 or more -- amounts that can be significant to campaigns in their early stages -- to Republicans, particularly those in close races, and to conservative Democrats. The $40,000 of Realtors' money going to moderate and liberal Democrats was almost entirely in amounts ranging from $200 to $500, contributions viewed less as expressions of support than as "courtesy" or "access" gifts.

The Sheet Metal and Air Conditioning Contractors have given $138,643 to Republicans and $27,051 to Democrats; of the money going to Democrats, $21,551 went to conservative supporters of the Reagan program. The Associated General Contractors reported giving $396,300 in direct contributions to Republicans and $86,900 to Democrats, although $50,000 of the money for Democrats went to Texas candidates for the House and Senate.

The conservative organizations. The conservative PAC community is split into two groups: One is the "old" right, represented by such organizations as the American Conservative Union and the Fund for a Conservative Majority, which concentrate more on issues of conservative economic policy and defense spending than on "social" issues. The other is the "new" right, including such groups as the Committee for the Survival of a Free Congress, the National Conservative Political Action Committee and Sen. Jesse Helms' (R-N.C.) National Congressional Club, which focuses on such issues as abortion, school prayer and busing.

During the primary season, the old and new rights took slightly different paths. The old right remained firmly in the GOP camp; the American Conservative Union specialized in running Washington fund-raisers for incumbents, almost all Republicans.

The new right entered a number of primaries with different goals, with mixed results, at best. For example, it failed to defeat Rep. Jack Brooks (D-Tex.) and also challenged the Republican establishment and the business community in some open-seat Republican primaries, unsuccessfully running a House candidate against Cissy Baker, daughter of Sen. Howard H. Baker Jr. (R-Tenn.), the majority leader, in Tennessee.

As the general election approaches, the old and new rights' inherent loyalty to the GOP is reasserting itself. The Fund for a Conservative Majority is targeting 13 Senate races and 72 House races; every candidate supported is a Republican.

The National Conservative Political Action Committee is preparing "negative" campaigns against six senators and eight House members; every target for defeat is a Democrat. The Fund for a Conservative Majority expects to be involved in about 55 general election races; a spokesman said that perhaps three of the candidates supported will be Democrats.

The independent oil PACs. Over the past four years, a network of political action committees financed primarily by independent oilmen has sprung up in Texas, Louisiana, Oklahoma, Colorado and Southern California. A key strategy of these PACs is to funnel money into non-oil states with the specific goal of defeating liberal to moderate incumbents. In practice, this translates into the channeling of large sums into Republican campaigns.

These groups include the Dallas Energy PAC (DALENPAC); the Louisiana Energy National PAC (LENPAC); the West Central Texas PAC; the Houston PAC; the Intermountain PAC (IMPAC); the Petroleum Exploration PAC of California; and about six smaller PACs in oil producing states.

Among this fraternity, DALENPAC offers the best example of concentrated political firepower, giving out $88,500 through June. Not only has every contribution gone to Republicans, but also, with only one exception, each gift was at least $2,000, and 10 of the contributions were the maximum $5,000. LENPAC contributed through May of this year $48,500 to Republicans and $14,500 to Democrats. All the money going to Democrats went to conservatives running in primaries, almost all of which are in districts certain to elect Democrats. IMPAC has given out $42,700, of which $40,700 has gone to 36 Republicans, almost all in tight races.