Senate and House candidates spent $374 million during the 1983-84 election cycle, a 9.3 percent jump over the previous election but considerably less of an increase than in the past, the Federal Election Commission reported yesterday.

By comparison, spending jumped 43 percent from 1980 to 1982 and 23 percent from 1978 to 1980.

The report showed that while the cost of winning the average Senate seat went up in 1984, the cost of winning the average House seat fell. Total spending in Senate races increased 23.1 percent in 1982, while spending in House races decreased .2 percent.

The FEC said the average House race cost less probably because there were more safe incumbents -- who had previously won with more than 60 percent of the vote -- than in previous elections. There was also a smaller number of House races not involving an incumbent. The open-seat races are usually the most expensive.

The 2,035 candidates who ran for the House and Senate in the two-year cycle collected a total of $398.6 million and spent $374.1 million. Political action committees contributed $104.9 million to congressional candidates, 26 percent of the money they raised.

Sen. Jesse Helms (R-N.C.) spent $16.5 million winning reelection in his tight battle with then-Gov. Jim Hunt (D). Helms set the previous record of $7.5 million in 1978.

John D. (Jay) Rockefeller IV (D-W.Va.), using much of his own money, spent $12 million in his successful Senate race. Hunt's losing effort in North Carolina was third, $9.4 million. Phil Gramm (R-Tex.) spent almost that much to win his Senate seat.