A committee set up to draw new congressional districts for Maryland has come up with a plan that will dramatically reduce congressional representation in Baltimore and slightly adjust districts in the Washington suburbs.

The new plan, which was given tentative approval today by the governor's advisory committee on redistricting and reapportionment, is the first step toward redrawing congressional lines in accordance with population shifts documented in the 1980 census.

Because the state showed a slight gain in population it will not lose representation in Congress, but the new districts will have to reflect a shift in population from urban to suburban and rural areas.

Although the final boundaries have not been decided on, the committee plan leaves Baltimore, which lost 13 percent of its population between 1970 and 1980, with only one district -- the predominantly black 7th District now represented by Rep. Parren J. Mitchell -- dominated by city voters. For political as well as constitutional reasons, the district must continue to be black but because it has lost so much population in the last 10 years it will have to be expanded west into Baltimore County.

The other city district, now represented by Rep. Barbara A. Mikulski, will be partially moved out of Baltimore to pick up needed population in Baltimore County and the Columbia area in Howard County now represented by western Maryland Rep. Beverly Byron. Currently about 80 percent of Mikulski's district is in Baltimore City but under the new plan, it would be reduced to half.

In the Washington suburbs, northern and western sections of Montgomery County around Poolesville and down to the Potomac, which are now represented by Rep. Michael Barnes, will be moved into Byron's western Maryland district. In neighboring Prince George's County, Rep. Steny Hoyer's 5th District, which is now 60,000 people below the ideal congressional district population of 527,056, will be moved south into the Clinton and Upper Marlboro area. That shift will place Hoyer's residence in his own district for the first time.

Although the new lines for the 5th District skirted some of the predominately black sections of southern Prince George's, Hoyer's new district will be 37 percent black, the second highest concentration of black voters of any district in the state.

Pushing the 5th District line south moves Hoyer into some Prince George's areas now represented by Marjorie S. Holt, the only Republican house member from Maryland. Holt's 4th District, which is centered in Anne Arundel County, had to lose some 60,000 people in order to come into line with the ideal population per congressional district.

The 1st Congressional District, which stretches from rural southern Maryland across Chesapeake Bay and into the farmlands of the Eastern Shore, is now some 82,000 over the proper size, and under the advisory committee plan will lose part of Harford County.

Harford, which like Howard and other suburban parts of the state has grown rapidly in the last decade, will be split in two and the western segment will be linked with Baltimore County and the 2nd District now represented by Rep. Clarence D. Long.

That shift, which has prompted some criticism in Harford County, means that Long's district will then have too many people. As a result, he will have to give to Mikulski some sections of Baltimore County -- either the ethnic working class neighborhood around Dundalk, east of Baltimore City, or the heavily Jewish areas around Pikesville, northwest of the city, both of which would like to stay in the 2nd District.

Which of the two areas will be moved and exactly how lines will be drawn in Baltimore are the only major unresolved issues facing the advisory committee.

The committee will give its final approval to the congressional plan on Nov. 17 and then forward it to Gov. Harry Hughes. Hughes will then have the amended plan introduced in bill form to the General Assembly next winter. Under the law any legislator can also propose a congressional redistricting plan.