The Republican Party, increasingly fearful of a Democratic takeover of the Senate, is preparing to channel millions of dollars into key states, breaking by far all previous levels of party support in a general election.

The total amount of cash dwarfs the level of Democratic Party support to its candidates. In close races, the money can prove critical to the outcome; in 1982, for example, Republicans won all five close Senate races, a result many offi- cials in both parties credit to GOP advantages in cash and high technology.

While publicly voicing confidence that the GOP will hold the Senate -- it now has a 53-to-47 majority -- many Republican officials and consultants gathered here for party meetings privately stress that there are 12 to 13 currently Republican seats that could turn into close races, while only four to five Democratic seats are likely to produce tight contests.

"The math is with them," one Republican strategist said.

Republican and White House officials attending a summer meet- ing of the Republican National Committee here described their plans:The GOP national and senatorial committees plan to put about $10 million into the local parties of 15 to 17 states with close Senate races. This money will go to "party-building" activities -- registration, direct mail, development of computerized voter lists, absentee ballot drives.

Republican strategists say this money will not count against contribution limits placed on the parties by federal election law, a stance that may be challenged by Democrats.

Federal law limits national party contributions to Senate candidates under a formula based on 4 cents per voter in the respective states, which translates to a maximum of about $1.7 million in California down to about $87,000 in South Dakota.

Republican officials say they can contribute notwithstanding those limits because additional money is permissible under provisions allowing contributions to programs encouraging the growth of state and local parties.

In private, Republicans say their problem is not raising money but deciding how to legally spend what they have.

The $10 million national party drive compares to a far more primitive $1.2 million program planned by the Democratic National Committee. Most of the DNC money will go to pay salaries of 32 campaign and finance specialists performing basic organizing duties for state parties. The GOP put basic organization in place in most states years ago.

As one part of the GOP effort, Frank J. Fahrenkopf Jr., RNC chairman, said the committee will send out 12 million pieces of direct mail and make 10.5 million phone calls to prospective voters. The DNC has no parallel plans. In terms of direct and indirect party contributions to Senate candidates, the Republican senatorial campaign committee plans to spend at least double the amount of the Democratic senatorial committee, $10 million to $5 million. At a level more difficult to trace, state Republican parties, particularly in the South, are raising large sums of money that they intend to use in support of all candidates, including Senate candidates.

In Louisiana, for example, the Sept. 27 election pits Reps. W. Henson Moore (R) and John B. Breaux (D) and some lesser candidates in a contest for the Senate seat. The state and national Republican parties plan to put more than $2 million in the race, over and above the amount given directly to Moore. The Democratic Party in Louisiana, after years of disorganization, is just beginning to rebuild and has almost no money to put into the election.

Terry Michael, spokesman for the DNC, said the Democratic Party has initiated a special program to raise money for state parties that so far this year has produced $255,000. In contrast, President Reagan raised twice that amount for the Louisiana GOP at just one fund-raiser earlier this year. Lanny Griffith, southern political director for the RNC, said state parties in Georgia, Florida, North Carolina and Louisiana will all set fund-raising records this year. The Republican Party cash advantage is generally matched or exceeded by the financial advantage held by Republican Senate candidates over their Democratic opponents.

An examination of reports from 15 close races showed the Republicans with a decisive -- $500,000 or more -- advantage over their Democratic opponents in 10 of the contests. In the remaining five, the candidates were roughly equal in three, and the Democrats lead in only two. Overall, the GOP Senate candidates in these states raised $26 million, compared to $18.4 million by Democrats.

Officials of both parties cite the success of the GOP in all the close 1982 Senate races as demonstrations of the importance of money and technology. In Missouri, for example, expensive daily tracking polls financed by the GOP Senate committee showed that the Democratic candidate in 1982, Harriett Woods, was pulling ahead of Sen. John C. Danforth (R).

The information persuaded the Danforth campaign to make a sharp switch in tactics, moving to negative attacks on Woods, who, in turn, was broke, and had no last-minute poll information on which to counter Danforth's strategy. Danforth barely won reelection.

On a broader scale, GOP officials contend their massive buildup of voter lists, phone banks and direct mail has allowed the GOP to overtake the Democratic Party, and its labor and civil rights allies, in terms of the total number of "voter contacts."

In 1982, according to post-election polling, pro-Democratic groups reached 1.5 million more voters than the GOP and its allies. By 1984, with well over $10 million invested by the GOP, pro-Republican voter contacts outnumbered Democratic efforts to persuade voters by over 1 million.