Spending for House election campaigns was lower in 1984 than in 1982 -- the first time in memory such a decline has occurred. In addition, fund-raising by congressional candidates last year slowed dramatically, barely outpacing inflation.

The seeming leveling-off comes after a string of elections in which House campaign receipts jumped enormously, by as much as 48 percent. Experts yesterday offered a number of explanations for the drop -- such as more incumbents seeking reelection and fewer closely contested races than in 1982 -- but some contended that the change was too sharp to understand without closer study.

Figures compiled by Common Cause, the self-styled citizens' lobby, showed that spending by the 802 House candidates on the November ballot dropped to $165 million, a decrease of 5.4 percent from the $174.5 million spent on 1982 House elections.

In addition, fund-raising, while reaching a new height, grew at a more sluggish pace than in the past, and some experts suggested that it may have leveled off. The 1984 total of $196.8 million was a 7.3 percent increase over the $183.4 million raised by 828 House candidates in 1982.

"The arms race may be over," said a student of campaign financing trends, Thomas W. Skladony of the American Enterprise Institute (AEI). "If you look at the rate of increase in previous years, from one election to the next, it had been going up by 40 percent, then 35 percent and then 48 percent. Now you have a rate of increase that's just about keeping pace with inflation."

Inflation has grown by 6.6 percent since 1982, Skladony said, adding that "candidates don't buy milk. The things candidates do buy, like TV time, have gone up faster than the CPI," the government's Consumer Price Index.

As a result of the drop in campaign spending, House candidates wound up the two-year election cycle on Dec. 31 with a record-setting $37.5 million cash on hand, 77 percent more than what remained at the end of 1982. Incumbents held most of the leftover cash, with 33 of them -- 18 Democrats and 15 Republicans -- each sitting on war chests of more than a quarter of a million dollars.

Incumbents also collected more than two of every three dollars raised, or $133.1 million, with 44 percent coming from political action committees. House members running for reelection in 1980 got 34 percent of their money from PACs; in 1982 the figure was 37 percent.

In its study, released yesterday, Common Cause said one reason for the drop in spending was that there were fewer open-seat races in 1984 -- 27 compared with 58 in 1982. Open-seat contests usually are more expensive, the study noted; in 1984 they cost, on average, 70 percent more than contests involving an incumbent.

Another factor, the group said, was the larger number of House candidates without opposition on the November ballot -- 64 compared with 46 in 1982.

Finally, it said, there were fewer "marginal" or closely contested races, where the victor won 55 percent or less of the vote. Common Cause said there were 57 of these traditionally expensive campaigns in 1984, compared with 82 in 1982.

Skladony said these factors help explain the lower rate of growth in fund-raising, but he added that a 7 percent increase after a succession of larger ones was too "dramatic" not to have other explanations.

Studies by Skladony and AEI fellow Michael J. Malbin show that House campaign receipts increased by 40 percent in 1978 (compared with 1976), 35 percent in 1980 (compared with 1978) and 48 percent in 1982 (compared with 1980).

"People are going to have to do some in-depth work to figure out the reasons," Skladony said. "Maybe the political parties are doing more for the candidates . . . . If so, that's encouraging, especially if you're worried about undue influence from PACs."

Common Cause President Fred Wertheimer took a dimmer view of the PAC outlays, especially to incumbents. "This system must change -- and change soon -- if we expect 'representative' government to survive," he said.

Despite the overall drop in House campaign spending, the Common Cause study showed that in 24 districts last fall, combined expenditures by the major-party candidates exceeded $1 million. And 35 House candidates -- 18 Republicans and 17 Democrats -- spent more than $600,000 each.