The more than 3,200 political action committees, on their way to their heaviest election-year spending ever, are dividing their money evenly between Democratic and Republican congressional candidates and heavily favor incumbents over challengers.

Campaign finance reports through Sept. 30 filed with the Federal Election Commission and analyzed by Common Cause, the public watchdog group, also show business PACs outspending labor PACs by better than 2 to 1 but distributing their funds in a more bipartisan pattern.

The review of spending and PAC receipts in all 33 Senate contests and 87 of the most hotly contested House races around the country shows that:

* Spending by major party candidates for the Senate had topped $80 million as of Oct. 13, with the three heaviest spending weeks remaining. In 1980, Senate candidates spent $75 million for the entire campaign.

* The biggest spender in a Senate race through Oct. 13 is department store heir Mark Dayton (D), who has sunk $5.7 million -- all but $200,000 from his own pocket -- into his effort to unseat incumbent David F. Durenberger (R-Minn.). The most frugal candidate has been Sen. William Proxmire (D-Wis.), whose reelection bid has cost $100.

* Incumbent senators seeking reelection have received an average of 24 percent of their money from PACs, with the rest coming from individual givers and party sources. Challengers have received 12 percent of their funds from PACs, open-seat candidates 12.7 percent.

In 87 House races considered by both parties to be battleground districts, PACs have provided 30 percent of funds raised, with incumbents receiving 35 percent of their funds from PACs, challengers 28 percent and open-seat candidates 20 percent.

Those percentages are expected to increase by Tuesday because a larger proportion of the contributions a candidate receives in the last month is PAC money.

In the decade since their development was spurred by election finance reform laws, political action committees have assumed an ever-growing role in the financing of congressional elections. In 1972, they accounted for about 13 percent of all congressional candidates' fund-raising, and by 1980 the figure had grown to 25 percent. This year, with the expected heavy infusion of late PAC money, the figure is expected to approach 30 percent.

PACs are organizations connected to business, labor and trade associations or ideological groups that raise money from like-minded members and contribute it to candidates. They can give a candidate as much as $5,000 in each election.

This year's campaign also has seen a dramatic increase in another type of PAC expenditure that did not become common until the late 1970s, the so-called "independent expenditures." This money is not given directly to a candidate but is spent by the PAC itself to influence the outcome of an election.

Through Aug. 31, independent expenditure groups had spent $3.35 million on congressional elections, about 50 percent more than they spent in the entire 1980 congressional campaign.

"An extraordinary amount, 94 percent, of independent spending is negative aimed at defeating a candidate ," said Frances Zwenig, a spokesman for People for the American Way, which has been studying such expenditures.

While some liberal ideological groups have joined the negative spending parade this year, the National Conservative Political Action Committee (NCPAC) is still by far the largest spender, accounting for 77 percent of independent expenditures.

In 1980, NCPAC's "attack ads" were widely perceived as contributing to the defeat of four prominent liberal senators. However, every campaign has a different dynamic, and this year the targets of such ads appear to have learned how to counterpunch more effectively.

In several states where NCPAC ads have appeared this year, the candidates presumed to benefit from the ads have wound up scrambling to dissociate themselves from the group.

Another general pattern in PAC giving this year in marked contrast to 1980 is that far more dollars are flowing to incumbents.

In 1980, business PACs especially became more adventuresome about backing challenger candidates. They broke from the lockstep pattern of paying "access" money to incumbents and began investing heavily in challengers imbued with the free enterprise ideology. In 1980, corporate PACs gave 30 percent of their funds to challengers, up from 20 percent in 1978.

This year, in Senate races with Republican incumbents and Democratic challengers, business PACs have invested in the incumbent by 60 to 1. In races with Democratic incumbents and Republican challengers, they have favored the incumbent by better than 2 to 1.

Indeed, the general pattern of the year has seen the business PAC community generally trying to protect vulnerable incumbents, while labor PACs have been more aggressive about backing challengers.

In years past, labor PACs were known to distribute a lot of what is known as "thank-you money," contributions to staunchly pro-union incumbents who face no serious opposition.

"We have made a much more concerted effort this year to target our money," said AFL-CIO spokesman Murray Seegar. A review of the 87 battleground House districts, for example, shows business outspending labor by less than 2 to 1. In contrast, in all 33 Senate races, which include several races not closely contested, the business spending ratio over labor grows to more than 2 to 1.

Unchanged this year is labor's affinity for Democrats: It has given to Democrats over Republicans in the Senate races by 93 to 7 percent and to Democrats over Republicans in the battleground House races by 99 to 1 percent.

Business has favored Republicans over Democrats in the Senate races by 67 to 33 percent and in the battleground House races by 85 to 15 percent.

The top five Senate spenders this year are Dayton, $5.7 million; California's Pete Wilson (R), $5.1 million; California's Edmund G. (Jerry) Brown Jr. (D), $3.9 million; Sen. Lloyd Bentsen (D-Tex.), $3.8 million, and New Jersey's Frank R. Lautenberg (D), $3.7 million.

Top Senate PAC recipients are Wilson, $816,311; Sen. Orrin G. Hatch (R-Utah), $681,570; Bentsen $664,927; Durenberger, $612,406, and Sen. Robert C. Byrd (D-W.Va.), $597,800.

Top Senate labor PAC recipients are Sen. Paul S. Sarbanes (D-Md.), $234,656; Byrd, $231,500; Sen. Donald W. Riegle Jr. (D-Mich.), $208,955; Brown, $204,500, and Sen. Howard M. Metzenbaum (D-Ohio), $204,170.

Top Senate business PAC recipients are Hatch, $574,845; Wilson, $563,811; Bentsen, $517,770; Sen. Richard G. Lugar (R-Ind.), $509,744, and Durenberger, $468,356.