It definitely pays to be an incumbent. One third of the 4,157 special-interest groups making political contributions in the 1986 congressional elections gave at least 80 percent of their money to incumbents, according to a new Common Cause study.

Contributions from all political action committees accounted for $138 million of the campaign funds, and 68 percent or $95.1 million went to incumbents, the study said. That is nearly five times the $19.4 million given to challengers. The remaining $23.6 million in PAC funds went to candidates for open seats.

Common Cause President Fred Wertheimer accused the PACs of "bending over backward to buy influence with incumbents and lock challengers out of the process."

Common Cause supports legislation that would limit the total amount of PAC money a candidate could accept -- there is no limit now. Several incumbents received more than $1 million in PAC money in last November's elections. Under election law, PACs are allowed to give $5,000 to a federal candidate per election.

The study said 20 of the 104 PACs with donations of $100,000 or more gave 90 percent or more of their overall contributions to incumbents.

Of the two largest contributors, the National Association of Realtors gave the most money to incumbents -- $2.2 million -- or 77 percent of all their contributions. The National Education Association, which donated more than $2 million, gave only 59 percent to incumbents.