A special committee agreed last night on a redistricting plan for the Prince George's County Council that may ensure the defeat next year of two incumbent Democrats, including Sue V. Mills, one of the most outspoken and colorful politicians in the county.

The plan, designed to win enactment by a majority of the current council, also would probably ensure the election of two blacks, although some members of the committee had pressed for a plan that would bring three blacks to the council, which will have nine members.

Besides Mills, the plan jeopardizes the seat of Ann Landry Lombardi, one of the council's most liberal members, who has specialized in health issues. Mills and Lombardi are in their first terms on the council.

The council currently has 11 members-- all Democrats -- but, because of a county charter amendment approved by the voters last year, it must shrink to nine after the next election. This situation sent the incumbents scrambling for support from the commission they appointed to draw new districts.

Mills has been considerably more popular with the voters than with her colleagues. The committee's plan would place her in an 80 percent black district in which another incumbent, Deborah Marshall, who is black, also resides. Mills' strong stance against school busing for desegregation -- which launched her to prominence a decade ago when she was on the school board -- and her conservative approach might doom her in such a race.

Mills said last night she was not surprised at the move and added: "I have no problem running against Debbie Marshall, but if they have lifted me from my base of support I shall run for county executive."

The council has 70 days from the time the districting plan is formally submitted, probably early next week, to amend it. If it fails to act, the plan becomes law.

The document was adopted by the committee last night with the votes of its three Democratic members over the opposition of its two Republicans. Its author, New Carrollton lawyer Thomas Hendershot, said it was drawn with the political futures of the incumbent Democratic council members in mind.

Originally, Hendershot and committee member Wayne Curry had sought to ensure election of three blacks to the new nine-member council. The current 11-member council has three blacks, one of whom is not considered reelectable, and the county's population is 37 percent black.

Hendershot and Curry said last night they found it impossible to draw three districts that would ensure the election of blacks without diluting the vote of middle-class black homeowners.