Maryland Democratic leaders, who want to buy a $100,000 computer to modernize the party's campaign operations, are raising membership fees paid by participating office holders and other candidates.

The party plans to offer services ranging from computerized voters lists with demographic details to survey research and media profiles.

Such data helps candidates more effectively target their mailings, commercials, and get-out-the-vote strategies.

At a press conference announcing the project, called Maryland Campaign Central, state party chairman Howard Thomas said, "We're saying to the Republican Party of Maryland, we're able to meet their challengers and send them off as losers."

Baltimore County Executive Donald Hutchinson, presenting the group with a $1,000 contribution, added, "We are the party that reflects the mainstream of Maryland. We must join forces in an effort to make sure it is our candidate that prevails."

Registered Democrats still outnumber Republicans three to one in the state, despite some recent modest gains and a couple of switches by well-known former Democrats, including former U.N. ambassador Jeane Kirkpatrick, a Bethesda resident.

The state's most prominent Republican, Sen. Charles C. Mathias, plans to retire and, even with former U.S. attorney George Beall considering entering the race, the GOP is uncertain of having a candidate with realistic chances of winning. Rep. Marjorie Holt is retiring, leaving Rep. Helen Bentley as the "There's clearly a switch back to . . . responsibilities being handled by state and local governments." -- Keith Haller only sure Republican member of the state's congressional delegation.

But the Democrats say they are looking beyond the 1986 elections to a party-building strategy that includes more help for candidates at the grass-roots level.

Chiefly, the Democrats say they want to counter the technical benefits many local GOP groups around the country received from the mammoth, $8 million organizing effort the Republican National Committee mounted in behalf of the Reagan-Bush campaign last year, including voter registration lists matched with demographic data.

Future GOP programs are expected to assist local GOP organizations in raising funds and other campaign activities. Some state GOP organizations have invested their own money to help come up with more detailed lists.

Though Maryland's GOP benefited little from that activity -- "We didn't have the money" to invest in very detailed data, said Republican Party Chairman Allan Levey -- the Democrats want to be sure to maintain their dominant position in the state by emulating the technical ability that has given their opposition an edge elsewhere.

Equally important, however, said Bethesda pollster Keith Haller, a consultant to the Maryland Campaign Central, is the need for Democrats to take seriously the quality of campaigns and races at the lower, less glamorous, level of the political arena. Haller, president of Potomac Survey Research, said recent polls show that voters are showing higher regard for local politicians than in previous years.

"There's clearly a switch back to the state and local level and a recognition that greater functional responsibilities are being handled by state and local governments," he said.

With the Democratic Party's national concern about its standing among so-called average voters, Haller said, local parties would do well to use newly available technologies to keep tabs on issues of local interest.

Party officials have yet to resolve, however, whether all Democrats, or just incumbent office holders, will have access to the information developed by the party.

And while pondering the question of how Maryland Democrats can best deal with their Republican adversaries, Prince George's Democrats, never known for delicacy in dealing with malcontents, have come up with a response to one of their own who recently jumped the fence to the GOP: They'll deprive him of his plum committee assignment in the General Assembly.

"I have no choice. It's the House rules," said House Speaker Benjamin Cardin.

Del. Thomas Mooney, a Takoma Park Democrat who changed his party affiliation last week for the GOP, citing the need for "intellectual honesty," will return to the General Assembly this winter without his seat on the prestigious Ways and Means Committee, which considers such important issues as tax legislation and funding formulas.

He is being assigned to the environmental matters committee, not exactly legislative Siberia, but close enough for those who cannot adjust to its diverse issues and strong, contentious personalities.

Prince George's delegation chairman Charles (Buzz) Ryan, who freely admits that he made sure to inform the speaker of his duties in the matter, insists that the change comes about only because members of the minority party must be equally distributed among all the committees. With Mooney, the Republican now have 18 members to spread among six committees.

Ryan says the move isn't punishment, but adds that Mooney "caused us some administative problems."