The Maryland Republican Party, in an effort to bolster its ability to pick winning candidates in this Democratic state, is considering selecting its nominees for state offices at a convention instead of in primaries.

The proposal would allow the GOP to endorse a slate of candidates for governor and three other state posts several months before party primaries, giving its candidates an early start.

Maryland Democrats hold a 3-to-1 voter registration edge that the Republicans have only rarely overcome in local, state or federal elections.

Under state election law the Republicans would still have to hold a primary, but those candidates with the party's stamp of approval would probably have a considerable edge, said state Republican Chairman Allan C. Levey.

Although Levey said he thought the convention plan, similar to that used in Virginia, would rejuvenate the underdog GOP, several Republican activists and officials attacked the proposal as an effort to increase the power of party regulars and curtail voter input.

"We are diluting the citizens' ultimate right in a democracy -- that is their right to vote," Helen Chamberlain, a member of the Baltimore County Central Committee, said at a GOP-sponsored public hearing today. "We need to continue to broaden our base and as far as I can see this new procedure is only diminishing that base."

Levey, who originally proposed the convention idea last spring, said the process of selecting convention delegates would increase grass roots interest in the party. Under the proposal, as many as 2,500 convention delegates would be selected in open caucuses at the county and district level.

The delegates would not necessary be committed to a candidate, Levey said, though in all likelihood each candidate would field a slate of delegates. The delegates chosen by the caucuses would then go to the statewide convention, which would endorse a candidate for governor, attorney general, comptroller and U.S. Senate.

Despite the worries of some Republican, the proposal is likely to be endorsed this week by a special committee set up by Levey. However, before any changes are made -- changes that Levey hopes will take effect this spring in time for the 1982 elections -- it must be approved by the full state party at a meeting in November.