Senate and House candidates spent $103 million on their primary, special and general election campaigns in the 18-month period ending June 30, the Federal Election Commission reported yesterday.

Although the FEC was unable to make a direct comparison with the same period in 1977-1978 and does not make projections, there were strong indications that congressional spending is soaring toward a new record.

The strongest evidence came from Richard P. Conlon, staff director of an organization of House Democrats called the Democratic Study Group, after a survey of about 25 House Democratic incumbents and of campaign specialists in the Senate.

Conlon's conclusion: the $103 million represents only about one-third of the total that candidates will have spent by the end of 1980.

His estimated final figure of more than $300 million compares with the FEC's adjusted totals of $194.9 million in 1977-1978 and the relatively small $99 million spent for the 1976 contests. The figures do not take inflation into account.

Conlon told a reporter that House campaign managers said they had spent only 10 percent to 33 percent of their expected year-end totals by June 30. The average was about 20 percent, compared to about 40 percent in the Senate, he said.

The FEC report, covering a total of 2,184 candidates who raised a total of $125.8 million after Jan. 1, 1979, shows:

Receipts from political action committees of $20.8 million, including $7.4 million from corporate PACs; $6.1 million from trade association units, which often favor the same candidates, and $5.3 million from labor PACs.

Contributions by Republican Party organizations of $1.5 million, five times the sum given by Democratic Party units.

Spending by House Democratic candidates of $30.4 million, or $5.1 million more than their GOP counter-parts, and by Senate Democratic aspirants of $25 million, $2.8 million more than their Republican rivals (the DSG said Tuesday, however that it expects this trend to be reversed by the heavy spending now under way).

Spending by incumbents of $41.9 million, by challengers of $38.7 million, and of $22.4 million by candidates for open seats.

The FEC also found that expenditures on behalf of candidates by PACs claiming no affiliation with those candidates came to $40,000. So-called independent spending by PACs in opposition to candidates came to $800,000.