Despite a lopsided 7-to-1 Republican lead over the Democrats in national party fund-raising, individual GOP candidates enjoy only a relatively modest edge in the races where money matters most -- the marginal contests for Congress.

An analysis of the mid-October campaign finance reports from 117 battleground congressional districts shows that the average Republican candidate in those races had spent $268,655 and the average Democratic candidate $215,311. For about 80 percent of these races the reporting period was through Oct. 12; the rest were as of Sept. 30.

These figures do not include two prominent forms of party-financed assistance that go unreported in the candidate filings: a national party advertising campaign, and a "coordinated expenditure" provision that permits the parties to pump a maximum of $36,880, generally for media buys or direct mail, into a congressional candidate's campaign.

Republican candidates are enjoying the fruits of these party resources much more than Democrats. But even allowing for them, the Democrats are holding their own in the marginal races better than party leaders had anticipated.

"It is surprising," acknowledged Martin Franks, director of the Democratic Congressional Campaign Committee, who has been complaining all year that the combination of national GOP money and targeted contributions from business political action committees would enable the Republicans to "buy down" their midterm election losses in the tight races.

Democratic National Chairman Charles T. Manatt has repeatedly called the national GOP fund-raising edge "obscene"--it stood at $168.8 million for the Republicans to $23.6 million for the Democrats through Sept. 30. He contends that it will cost his party six to 10 seats it would otherwise pick up.

But the Democratic candidates in close races appear to be the beneficiaries of two fund-raising trends this year. Labor and liberal PACs are targeting their money more heavily than ever into marginal races, and the business PACs appear to have adopted a strategy of protecting incumbents of both parties than electing Republicans.

For the first 18 months of the 1981-82 election cycle, 93 percent of labor PAC money -- $8.1 million -- went to Democrats, while corporate PACs gave $7.12 million -- 57 percent of the $12.5 million they have contributed -- to Republicans and $5.38 million to Democrats.

However, it is probable that in the final weeks of the campaign, a large block of business PAC money has and will continue to go to Republican candidates in tight races.

That was the pattern in 1980 when, according to a study by the Democracy Project, a liberal watchdog group, GOP candidates in the 60 closest House races received more than one-fifth of all their contributions from business PACs in the last 20 days, when public disclosure of the gifts wouldn't be reported until after the election.

"I'll feel a lot better about the money figures if they show up the same way as they do now after Nov. 2," said Franks. "My sense is that there's a lot of business PAC money out there that is still moving around."

In the Senate, the mid-October reports in 18 of the most closely contested races show Democrats even more competitive in fund-raising than in the close House races.

The Senate aggregate totals are skewed, however, by the lavish campaigns of two Democratic multi-millionaires, department store heir Mark Dayton in Minnesota and computer entrepreneur Frank Lautenberg in New Jersey.

Dayton, the leading spender among all federal candidates this year, reported $5.7 million in expenditures through Oct. 12, most of it his own money. Lautenberg spent $3.1 million through Sept. 30, also relying primarily on his personal wealth.

In 18 of the most closely contested Senate races this year, Republican candidates have spent $1.56 million on the average and Democratic candidates $1.39 million.

Here again, the figures do not include the so-called party "coordinated expenditures made by the national and state party committees. In the Senate races, these expenditures can be as much as $1.3 million in California, ranging down to $73,000 in the least populated states.

The GOP has spent $7.1 million on such coordinated expenditures, providing candidates with the maximum allowable contribution in all but a handful of races. The Democrats have spent $1.6 million.

In both the Senate and House races, the most distinct pattern is not the partisan breakdown but the difference in fund-raising success between incumbents and challengers.

Democratic challengers in House races are the most disadvantaged. Of 46 such races reviewed, the average expenditure for GOP incumbents was $302,376 compared to just $161,682 by the Democratic challengers. Democratic incumbents in these close races have outspent their Republican challengers by an average of $275,307 to $220,217.

And in 29 open seat House races reviewed by The Washington Post, where neither has the advantage of incumbency, the GOP edge is relatively small--$249,758 to $215,889.

Strategists argue endlessly about the marginal value of campaign money beyond a certain threshold point. According to Manatt, "We can afford to be outspent 3 to 2 in these races, but not 3 to 1." CAPTION: Picture 1, Entrepreneur Lautenberg: $3.1 million.; Picture 2, Department store heir Dayton: $5.7 million.