he U.S. Justice Department today approved a Virginia House of Delegates redistricting plan that civil rights groups say will significantly increase black voting strength in the state.

The Justice Department action, required under the Voting Rights Act, is considered a major step toward ending a year-long dispute over legislative boundaries that had pitted senior legislative leaders against the state's black community and the federal government. Charging that they discriminated against blacks, Justice rejected two other plans.

But the plan, passed at a special legislative session April 1, faced new opposition this week. Three rural counties filed lawsuits charging that the new plan violates constitutional requirements that legislative districts be compact and composed of areas with a community of interest.

The counties--Middlesex, Augusta and Fauquier--are expected to be joined within the next couple of days by Rockingham and Christianburg counties and possibly others, according to a source close to the litigation. The suits are expected to make little headway, particularly if their claims are pressed in state court where the judges must be reappointed by the General Assembly every seven years, the source said.

"There isn't a judge in this state that would buck the General Assembly by striking down the redistricting plan if they have to get reappointed by the legislature," said one lawyer active in the redistricting litigation.

Meanwhile, the American Civil Liberties Union, which has been leading the fight against the legislature's redistricting plans on racial grounds, said it would drop out of a longstanding suit in federal court, where one redistricting plan had been rejected as unconstitutional. The ACLU along with the NAACP and other civil rights groups have proclaimed the latest plan--the sixth to be passed by the legislature over the last year--a victory for its cause.

The new plan divides the state into 100 single-member districts, abolishing the state's traditional practice of electing delegates from citywide, at-large districts that critics say have diluted black voting power. Nine of the districts, mostly in the cities of Richmond, Norfolk, Portsmouth and Newport News, have black majorities and civil rights groups say this could lead to the election of at least two more black members to the house this fall. Currently, there are only four blacks in the 100-member House.

Despite the proclamations of the civil rights groups, some Democrats have charged that the big winners under the new plan would be the state's Republicans. In carving out the inner-city black enclaves, the Democratic-controlled legislature was forced to create surrounding districts that are predominantly white and conservative. Some political analysts have predicted this could add six to 12 new Republicans to the 33 now in the House.

Besides the three redistricting plans rejected by the court or Justice, two others were turned down by former Republican Gov. John N. Dalton. The entire process, including 14 special sessions and bills paid to a Richmond law firm to defend the state in court, have been estimated to have cost taxpayers $1 million.