Corporate special interests, criticcized in the past for deserting their natural allies by donating to Democrats, shifted their contributions more to Republicans in the final weeks before last fall's election, according to Federal Election Commission records.

The shift wasn't a particularly dramatic or unexpected one. But it was considered significant because it came at a time when Republican National Chairman Bill Brock was complaining that corporate political action committees, by giving heavily to Democrats, were freezing Republicans into a permanent minority party status in Congress.

Last week, Republican presidential hopeful Ronald Reagan told a gathering of businessmen that, to incumbent Democrats, corporate groups were ensuring only that "the alligator will eat you last."

Until last Sept. 30, corporate political action committees (PACs) were making 53 percent of their donations to Republicans and 47 percent to Democrats, most of them incumbent congressmen and senators, according to an analysis of spending patterns by National Journal, a weekly magazine.

But during the next three weeks corporate interests pumped $2.9 million into Republican campaigns, while donating $1.2 million to Democrats.

This meant that by Oct. 23, the last date for which figures were available, corporate interests had contributed 61 percent of their campaign funds to Republicans and 39 percent to Democrats.

Corporate PACs, however, were still unable to offset the edge labor contributions have given Democrats in raising money from special interests. By Oct. 23, Democrats had received a total of $18.2 million from special interest PACs and Republicans $13.7 million, according to the FEC.

Labor, which donated 95 percent of its $9.3 million in contributions to Democrats, was the key factor here. Trade and professional groups, such as the American Medical Association, donated 56 percent of their funds to Republicans and 44 percent to Democrats.

PACs are the fastest-growing element in political financing. Three years ago there were fewer than 600. Today there are 1,910, with 812 representing corporations.

Most of the growth has come in corporate PACs, which until three years ago were in a legal gray area. Since then, their number has increased sixfold. This has raised fears among Democrats and public advocacy groups like Common Cause that corporations are on the verge of taking over campaign financing.

In the last two years, political action committees have raised $75.6 million and have donated $31.9 million to candidates. Most of the rest has gone to fund-rasing costs and overhead.

During last fall's campaign, PACs concentrated their contributions on House races instead of Senate ones, apparently on the theory that their donations would have a bigger impact there. By law, PAC donations are limited to $5,000 per election for any candidate.

PACs donated a total of $23 million, or 72 percent of their funds, to House candidates, helping these candidates, many of whom had only token oppostion, build up campaign surpluses totaling $11 million.

Rep. James M. Collins, (R-Tex.), built up a surplus of $261,625; Rep. Gillis W. Long (D-La.) a $155,371 surplus; Rep. Joseph G. Minish (D-N.J.) a $121,939 one; Rep. Bill Frenzel, (R-Minn.) a $103,537 one, and Rep. M. G. (Gene) Snyder, (R-Ky.), an $85,283 one, according to FEC records.

These surpluses are controversial because some congressmen and senators in the past have converted them to their personal use after they retire, a move that isn't prohibited by law, but which brings that income under provisions of the federal income tax law.