A report yesterday about 1983-84 campaign financing in Senate and House races incorrectly said the figures covered 18 months. They covered 21 months, from Jan. 1, 1983, through last Sept. 30.

Senate and House candidates already have raised more than a quarter of a billion dollars for what promise to be the most expensive congressional elections ever conducted.

Two studies released yesterday by Common Cause show the 65 Senate contenders on next month's ballot raising $112.8 million through the end of September, while House candidates in the nation's 435 congressional districts collected another $138.7 million. The figures cover 18 months.

Political action committees (PACs) contributed $68.9 million, more than 27.4 percent of the $251.5 million total.

House candidates received $47.9 million in PAC money, 34.5 percent of their total receipts. Senate candidates collected $21 million from PACs, 18.6 percent of their total receipts.

The biggest war chests, according to the Common Cause studies, were built up by two senators who are expected to win by landslides, Sens. Bill Bradley (D-N.J.) and Sam Nunn (D-Ga.), and a third, J. Bennett Johnston (D-La.), who already has been reelected.

Johnston won a third term on Sept. 29 and reported cash on hand of $1,462,959 as of Sept. 30. Bradley had $1.6 million at his disposal for the final month's campaigning; Nunn had $829,294.

In the House races, Common Cause reported, 25 candidates, all of them incumbents, each had more than $300,000 in the bank at the end of September. The leaders were Reps. W. Henson Moore (R-La.), $655,658; Stephen J. Solarz (D-N.Y.), $607,991; and Dan Rostenkowski (D-Ill.), $576,037.

Moore, like Johnston, won reelection last month. Rostenkowski, the influential chairman of the House Ways and Means Committee, and Solarz, who is looking for his sixth term, are considered shoo-ins.

Spending also hit new highs, according to Common Cause, with House candidates having paid out $100.7 million through the end of September and Senate candidates, $95.5 million.

In the presidential race, the Reagan-Bush campaign and the Mondale-Ferraro committee filed even more updated reports yesterday, covering expenses through Oct. 17.

The Reagan-Bush organization reported net spending of $25.6 million in public funds since the Republican convention and cash on hand of $15.2 million for the final stretch. The Mondale-Ferraro campaign said it had expenses of $26.5 million and cash on hand as of Oct. 17 of $13.3 million.

Three Senate candidates already have spent more this year than the leading spender of 1982, Mark Dayton of Minnesota, who spent $7.1 million in a losing campaign.

With the final and most expensive month still to go, Sen. Jesse Helms (R-N.C.) reported spending $12.8 million since Jan. 1, 1983; Gov. John D. Rockefeller IV (D-W.Va.), $7.8 milllion, and Helms' opponent, Gov. James B. Hunt (D-N.C.), $7.2 million.

In the House, Common Cause reported, 15 candidates, 10 of them incumbents, have spent nearly $500,000 each. At the top of the list are Rep. Joseph P. Addabbo (D-N.Y.), who won a vigorously contested primary last month, with expenses of $832,695; Rep. James Jones (D-Okla.), who faces a strong challenge, $768,703; and Manhattan Borough President Andrew Stein (D-N.Y.), who is challenging Rep. Bill Green, $753,489.

The North Carolina race between Helms and Hunt is the most expensive Senate campaign in history, its $19,995,021 price tag as of Sept. 30 already having eclipsed the $12.4 million record set in 1982 in California by Sen. Pete Wilson (R) and Gov. Jerry Brown (D).

The House races between Stein and Green (R-N.Y.) in Manhattan's Silk Stocking district, and between Jones and Oklahoma Republican Frank Keating in Tulsa, have already cost more than $1 million each. Two others, one in California and one in Oregon, have passed the $900,000 mark.

The Federal Election Commission confirmed in another study released yesterday that PAC spending has skyrocketed. Political action committees, the FEC said, contributed $57 million to federal candidates in the 18-month period that ended June 30, a 50 percent increase over the comparable 1981-82 figure.

Corporate PACs, which now number 1,763, led the way with $21.5 million in contributions; trade associations, health groups and other membership organizations were next with $13.6 million, and labor union PACs were third with $12.7 million.