For the first time, incumbent House Democrats received more campaign money last year from political action committees (PACs) than from individual donors, according to a study by the Democratic Study Group (DSG).

The dependence of House Democratic incumbents on PAC contributions was signficantly higher than it was for incumbent Republicans.

In addition, the study by the liberal-reform organization showed that the small contributor -- the donor of less than $100 -- is steadily disappearing from the political scene.

Donations of under $100 accounted for 46 percent of total House contributions in 1974 but 19 percent in 1984, while falling from 38 to 23 percent for Senate candidates over the same period. Even when inflation is taken into account, small contributions have declined by 17 percent from 1980 to 1984.

Just as House Democratic incumbents are more dependent on PACs than their GOP counterparts, the Democratic Congressional Campaign Committee and Democratic Senatorial Campaign Committee raise far more money from PACs -- 25 percent of their total revenues -- than the GOP committees -- less than 1 percent from PACs.

"PACs are displacing individuals as the primary source of campaign funds," the study concluded. "This problem is particularly acute with respect to incumbent members of the House, especially Democratic incumbents . . . the decline of small contributors cannot help but have a profound influence on Congress -- especially when grass roots-based funding is replaced by funds from interests outside of the member's district."

The DSG called for legislation granting a 100 percent tax credit for political contributions up to a total of $100 a year. The Democratic-controlled House Ways and Means Committee, however, last week moved in the opposite direction, voting to kill the current 50 percent tax credit on political contributions, up to an annual credit ceiling of $50.

Among the patterns:

*House -- Democratic incumbents in 1984 received an average of $151,202 from PACs and $144,868 from individuals. Republican incumbents received an average $128,474 from PACs, and $174,402 from individuals. All nonincumbents received significantly more from individuals than from PACs.

*Senate -- Democratic incumbents received a higher percentage of their contributions from PACs, 28 percent, than Republican incumbents, 21 percent. Total receipts for Democratic incumbents, however, were on average far less than for Republican incumbents, $1.9 million compared with $3.24 million. In terms of actual dollars, PAC contributions to Democratic incumbents were less than PAC gifts to Republicans, $540,772 to $676,570. For nonincumbents, there was very little partisan difference in the pattern of campaign contributions.