National Republican party organizations have parlayed their skill at direct mail fund-raising with special provisions of recently enacted campaign finance "reform" laws to become the major sources of funds for GOP House and Senate candidates around the country.

During the past 18 months, the GOP mass mail fund-raising operations have collected $39 million - a record for a non-presidential election period. The Democrats have come up with a measly (by contrast) $11.7 million.

With their funds, the Washington-based GOP groups have begun to pour an unprecedented $6.5 million into House and Senate races this year. The Democrats are budgeting only $2 million for congressional contests.

Recent campaign financing laws limit the amounts individual donors and political action committees (PACs) can give to congressional races, but leave an opening for national party organizations to take on more fund-raising activity. Ironically, the main impetus for these changes came from liberal Democrats, but none of them believed the party groups would raise the amounts the GOP now has.

Because of the way the laws were written, GOP groups may end up giving more money in individual races than big donors did in the past.

In Texas, to take the prime example, the Republican Senatorial Campaign Committee has already paid $189,000 worth of advertising bills for the reelection campaign of Sen. John G. Tower. This same committee has also given Tower $15,000 in cash and expects to pick up at least another $50,000 of his bills.

In Maine, the Senate GOP campaign committee has aided Rep. William S. Cohen, paying $30,000 in bills to help unseat Sen. William D. Hathaway, and one committee aide said it may give him $20,000 more.

Democrats, whose national party organizations have been far less successful than the GOP's in raising funds, tried earlier this year to take away the Republican advantage by another change in the financing law. But Republicans in the House were able to preserve their advantage by blocking the change.

The Republican groups have been raising most of their money in relatively small contributions generated through sophisticated, computerized mass mail campaigns.

Two things are extraordinary about the national GOP fund-raising efforts: no political party has ever raised as much in a nonpresidential year; neither has a party diverted so much money to congressional elections.

Whether the new centralization of fund distribution will be followed by some centralization of power within the party remains to be seen.

Under the 1974 and 1976 laws, individual contributors are limited to giving $1,000 to a candidate in any campaign, and political action committees set up by unions, corporations or special interest groups are limited to $5,000.

But the laws are looser for national and state party groups.

Party Senate campaign committees, for example, can give up to $17,500 to each candidate.

More important, national and state party committees can contribute on sliding scales that allow hundreds of thousands of dollars.

That's because the campaign law permits national and state party organizations each to pick up candidate expenditures up to a limit set by multiplying the state's voting age population by 2 cents.

Thus, in Texas, both the national and state GOP party organizations can pay Tower expenses up to $215,640.34. The Federal Election Commission has also ruled that, when the state party doesn't have the funds, the national organization can be designated to replace it. This will allow the national groups to give Tower $215,000 more.

One other unusual feature of the law is that neither the candidate nor his campaign committee has to report the expenditures picked up by the party organizations. Thus, the latest Texans for Tower campaign spending report lists some $939,224 raised, but that does not include the $189,000 in bills taken care of by the GOP Senate Committee.

Overall, the Republican Senate Committee now plans to spend about 2.3 million on 1978 races, almost 4 times the amount it distributed in 1976.

In several key races, the GOP group has already paid out its national spending allocation and has begun to pick up the state limit.

For GOP Sen. Jesse A. Helms' campaign in North Carolina, the GOP Washington organization in June paid a $94,000 television bill. The North Carolina limit is $94,977.

Three nonincumbent GOP candidates also have already gotten the limit in states where the Washington money could be significant.

In addition to Cohen in Maine, Rep. William L. Armstrong (R-Colo.), running against Sen. Floyd K. Haskell (D-Colo.), and Rep. Thad Cochran (R-Miss.), running for the seat being vacated by Sen. James O. Eastland (D-Miss.), have gotten the full national allocation of funds.

The National Republican Congressional Committee also plans to step up its already active Washington fund distribution. By law it can give only $5,000 in cash to a candidate for an election. Also, in House races national and state party groups can pick up a candidate's bills, to a limit of $12,290 per race.

This year the GOP congressional group has budgeted $3 million for distribution, almost double the amount it spent in 1976.

For most House incumbents, cash distribution of $4,000 in the primary and $4,000 for the general election has been made. In some 60 key races, additional bills are being paid up to $11,000.

The GOP House group, according to aides, had about $1 million on hand to be spent before election day.

Supplementing House and Senate GOP campaign committee fund activities is the work of the Republican National Committee, which has set aside about $1 million for spending on House races, mostly for nonincumbents, and another $150,000 for Senate campaigns.

The RNC is focusing on some 50 key races so that, along with the House GOP group, about 110 of 200 potential swing House races will receive large chunks of national GOP money.

In contrast with the GOP effort, the Democratic Party appears to have only a minimal program. Its House Campaign Committee plans to distribute about $1 million, its Senate group around $500,000. Recently Democratic Party Chairman John White announced that he hoped to spend about $500,000 on House and Senate races.