Political action committees set up by corporations, unions and other interests will spend between $55 million and $60 million on congressional races in 1980, or 70 percent more than in 1978, according to projections released yesterday by an organization of House Democrats.

The increase represents "a quantum leap . . . in the influence of special interest groups in the legislative process," the Democratic Study Group said.

The DSG estimate assumes that PAC receipts and contributions in the second half of 1980 will be in the same proportion to the year's totals as in 1978.

The DSG rejected as "premature" published speculation that corporate PACs, which as of June 30 had given nearly as much to Democrats as to Republicans, have abandoned a strategy of trying to defeat Democrats in order to "buy into" the majorities they may preserve on Capitol Hill.

The more likely scenario is that business PACs, as in 1978, will contribute heavily to GOP challengers seeking to defeat Democrats with a precarious grip on their seats "while continuing to buy access to those Democrats who have safe seats and who are generally responsive to business concerns," the DSG said. "In other words, help the healthy and shoot the sick."

Compared with 1978, the DSG found, corporate PACs have prospered more than any other category. As of June 30, they had raised, contributed and retained in cash more than twice as much as they had at the same point in 1978. By end of 1980, by the DSG's projection, they will have given congressional candidates more than $21 million -- more than 120 percent of the 1978 total.

By contrast, labor PACs have had the slowest growth, with receipts in the first six months of 1980 only 30 percent higher than for the same period in 1978.

DSG's chairman, Rep. David R. Obey (D-Wis.), cosponsored a bill to limit PAC money in House elections. The House passed it, but what the DSG called "a Republican Party/special interest coalition" has stymied it in the Senate.