A little-noticed special election contest here last spring reflected one of the a lasting advantages of President Reagan's reelection campaign that goes beyond the president's popularity.

With control of the Michigan Senate at stake, the Republicans maintained their one-seat majority in the March election not only by using their huge financial resources, but by capitalizing on the high-tech legacy of the Reagan campaign.

What the Reagan campaign left the Michigan Republicans, and state parties elsewhere around the nation, were extensive computerized lists of voters, categorized by demographic groups. That gave the Michigan Republicans an advantage over the Democrats in targeting their direct mail appeals and in mobilizing their supporters to get to the polls on Election Day.

"It's really the most important legacy the Reagan campaign left the party," said Lee Atwater, a Republican consultant who was deputy campaign manager of the Reagan-Bush '84 Committee.

The millions of dollars the Reagan campaign committee poured into the development of voter registration lists is a legacy the GOP plans to use to further its efforts at party building.

The lists will be used not only in the 1986 midterm election, but also in the GOP's longer-term effort aimed at gaining enough state legislative majorities by 1990 to give them an advantage in the redistricting wars that will follow the 1990 census.

None of this could be done without the GOP's huge financial resources. As the final returns came in here last March, Republican National Chairman Frank J. Fahrenkopf Jr. got a call from one of the RNC field operatives with the good news that Republican Vern Ehlers was heading for a 10 percentage-point victory.

After a pause, according to the aide, Fahrenkopf asked how much the Republican Party had put into the contest. Half a million dollars, he was told. How much of that was RNC money, Fahrenkopf asked. Sixty thousand dollars, came the reply.

Said Fahrenkopf, only half in jest but also reflecting his tightfistedness, "I don't mind buying an election, but I don't want to buy a landslide."

Despite their fat bank account, the Republicans are not prepared to buy landslides. Instead, they are designing specific programs geared both to the 1986 midterms and contests farther into the future.

Among GOP plans are:

*Channeling at least $1.4 million from the national Republican organizations (Republican National Committee, National Republican Senatorial Committee and the National Republican Congressional Committee) to the state parties to set up local fund-raising programs. If successful, the money raised by local parties in 1986 can be used to finance mailings and volunteer programs for House and Senate candidates and the money will not count against party contribution limits, according to RNC officials. (Under the law, federal party committees are allowed to spend about $60,000 on a House race and larger amounts on a Senate contest, depending on the state's population.)

*Targeting 20 to 50 Democratic-held congressional districts by the NRCC, which plans to spend $3 million to $6 million on generic ads to promote Republican positions and criticize Democratic leaders such as House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.). Republican leaders say this money will not count against party contribution limits because it is not aimed at defeating a Democratic candidate or promoting a specific Republican challenger.

*Funneling money into 50 other congressional districts where Democratic incumbents are entrenched but where voters have shown a willingness to support Republican presidential and statewide candidates. The money, to be spent by GOPAC, the political action committee headed by former Delaware governor Pierre S. du Pont IV, is described as "venture capital" for future party development. The GOP will aim this money at such places as Dade County, Fla. (Miami); Austin, Tex.; Camden, N.J., and the Chicago suburbs.

In addition, the GOP will continue to make extensive use of survey research data to expand its knowledge of voting behavior and patterns in key districts around the country. The NRCC, for example, plans to finance polls in at least 200 districts. At the same time, the RNC is paying $160,000 a month to Decision Making Information, the polling firm headed by Reagan pollster Richard B. Wirthlin.

Combined with the voter registration lists, the survey data can be a critical advantage in closely contested elections.

The Reagan-Bush Committee and the RNC poured in excess of $8 million into voter registration and get-out-the-vote drives in 1984. The cash was used in many states to build computerized lists of voters that merged households with voting histories; age, income and education information from the census; telephone numbers, and poll data.

Here in Michigan, the state Republican Party spent more than $600,000, largely through Market Opinion Research, the polling firm headed by Republican pollster Robert Teeter, to put together a comprehensive list of 2.2 million households.

Democrats concede that this gives the GOP an edge. "There are probably many states where the Republicans garnered a competitive advantage," said Richard Wiener, state Democratic Party chairman and head of the Association of State Democratic Chairs.

One irony of 1984 is that the Democratic Party's efforts to identify and register millions of new Democratic voters appear to have had only a fleeting impact on party building.

For one, the Democrats lacked the money to build up their lists early in the year, said Tim Finchem, who coordinated the distribution of millions of dollars for voter registration drives for Walter F. Mondale's presidential campaign.

Republicans began testing their voter registration lists in March 1984, but the Democrats did not start to distribute money for voter registration until mid-August, Finchem said.

The Democrats also failed to assure that the voter lists became the property of the party. "What happens on the Democratic side is that we are campaign-oriented, not party-structured," Finchem said.

In other words, Democratic voter registration money often went to specific Senate and House campaigns, where the information gained was discarded after the election. "You have to have continuity of management and funding" to achieve permanent gains for the party, Finchem said