The Republican Senate Campaign Committee is pouring an extra $900,000 into a series of tight Senate races in the closing days before Tuesday's elections.

Targeted at races where it could tip the balance, the money will increase the committees contributions to upwards of $3.2 million, or more than six times as much as its Democratic counterpart has given Democratic candidates.

The major recipients, according to reports filed with the secretary of the Senate, are incumbents Robert P. Griffin of Michigan, Edward W. Brooke of Massachusetts and John G. Tower of Texas, and former representative Jim Martin, who is seeking to succeed Maryon Alien in Alabama.

Each had received more than $143,000 by Oct. 28. Each is locked in a close battle for political survival.

The committee has also funneled large sums into other tight Senate races where Republicans have a chance. Among them are those in Colorado, Maine, West Virginia, Montana, North Carolina, South Carolina and Virginia.

But the most controversial donations were those to Martin, who expects to receive more than $230,000 by election day. And they may lead to a wholesale assault on the GOP committee's method of operation.

Supporters of Martin's opponent, Donald Stewart, have filed a formal complaint with the Federal Election Commission charging that the $194,700 Martin had received by Oct. 28 exceeded legal limits by more than $62,000. The GOP committee, the complaint alleges, "has made a mockery of the limits . . . It has unleashed a flood of funds well in excess of the lawful limit, an action which expresses its disregard of the law and undermines prospects fora fair election.

A Senate GOP campaign committee spokesman yesterday dismissed the complaint as a "classic case of sour grapes," and said it resulted from a failure by Steward, who has borrowed $250,000 to keep his campaign going, to generate financial support.

The complaint, however, raises an important legal issue that may affect the amount national party organizations can funnel to candidates in the future.

The issue is an outgrowth of campaign finance "reforms," which were pushed into enactment by Democrats but have turned out to benefit Republicans.

The key changes were ones designed to make political party structures stronger. They make it possible, for example, for party Senate campaign committees to give up to $17,500 to each candidate.

But more significantly, national and state party organizations are each allowed to pick up candidate expenditures up to a limit set by multiplying the state's voting-age populations by 2 cents.

Thus, in Alabama, both the state and national GOP organizations can each pay expenditures up to $62,506.94, those in Texas, up to $215,640.34.

Republicans, who have out-raised Democrats by more than 4 to 1, have interpreted the rule in a way that enables state party organizations to turn over their spending authority to the GOP Senate committee. This enables them to donate 4 cents per voter, or, in effect, to double their contributions.

Federal election officials have never ruled precisely on whether this is legal, although the GOP claims it received informal approval. Democrats claim Republicans are making an end run around the law and indicate they will press the issue.

This may take months, or even years, to resolve. Meanwhile, key Democratic strategists fear the donations may tip the balance in several key states.

Nowhere is the issue more clearly drawn than in Alabama, which has two Senate seats on the ballot this year. Martin originally entered one race, then on Oct. 2 switched to the other, feeling he had a better chance of winning it. GOP spokesmen maintain that because he has been in two different races he is eligible to receive twice as much as he normally would - or four times as much as the Democrats say he is entitled to.