Virginia Attorney General J. Marshall Coleman, who is attempting to tie his gubernatorial campaign to Ronald Reagan's popularity, broke with the administration yesterday, disputing the Justice Department's claim that a Virginia state senate redistricting plan discriminates against blacks.

Coleman said that since he became the state's top legal officer in 1978 he had "not observed any discrimination in the application of our voting laws." A Justice spokesman later noted, however, that the department had rejected two other changes in Virginia elections laws during Coleman's tenure because the proposals would diminish black voting strength in violation of the 1965 Voting Rights Act.

Both Coleman and his Democratic opponent, Lt. Gov. Charles S. Robb, said in a joint inteview that they oppose extension of the voting act, which allows Justice to reject proposed election law changes, including redistricting plans, in 22 states.

James P. Turner of the department's Civil Rights Division told Coleman's office last week the Virginia plan was drawn "with full awareness and expectation that it would fragment the black electorate and create two majority white districts" in the city of Norfolk, where blacks have never elected a state senator although they account for one-third of the city's population. Turner added that Virginia officials had presented "no plausible nonracial justification" for the plan.

But Coleman disagreed sharply, saying "I don't see any basis for saying there's a racial motivation. . . . My understanding of the law is that there is no obligation to carve out a black district. . . . "I don't think that the law says you have a violation of the act unless there's a racial motivation."

His interpretation immediately was contested by both Justice and civil rights advocates.

Carl W. Gabel, director of the Justice Department's voting rights section, said, "the act considers both effect and intent. And the courts have upheld objections on both."

The American Civil Liberties Union's Virginia spokesman Judy Goldberg said Coleman's statement "shows a shocking lack of knowledge of a major piece of federal legislation that has been in effect since 1965."

Virginia is one of six Southern states which, because of previous racial discrimination, has been required by the voting act to win clearance from Justice for any changes in its voting laws.

Coleman and Robb said in a meeting with Washington Post editors and reportes that their objections to extension of the act are limited to the clearance section, which expires next year. Sections of the act which ban poll taxes and literacy tests are permanent.

Justice Department spokesman John V. Wilson noted yesterday that the federal government has intervened only 14 times in the 2,930 Virginia election law changes proposed since 1965, the smallest number of interventions in any of the originally covered states.

But Goldberg and other civil rights advocates contended that the clearance provision had prevented many Virginia authorities from adopting voting changes that they knew the Justice Department would reject.

The Rev. Curtis Harris, president of the Virginia chapter of the Southern Christian Leadership Conference, said yesterday that Justice's rejection of the state senate redistricting plan "is a clear and unquestionable reason for extension" of the voting act.

Although Coleman said yesterday that he was not consulted on the redistricting plan, spokesmen for both his office and for Gov. John N. Dalton said Coleman had told the governor the senate plan was "legally defensible" before Dalton signed it in April.

Critics of the plan contended during its adoption that the boundary lines were drawn from north to south across Norfolk to protect the city's incumbent white Democratic senators. "Incumbency was probably foremost in the senators' minds, not race prejudices," said Norfolk Del. William P. Robinson Jr., a black Democrat, "but the effect was the same."

Justice's Turner noted in his letter that the legislature "rejected an alternative configuration that would have combined contiguous black neighborhoods, producing a district in which black persons would have constituted a majority."

That plan, offered by State Sen. L. Douglas Wilder (D-Richmond), the only black in the 40-member Virginia senate, would have drawn east-west boundary lines in Norfolk.

Justice spokesman Wilson noted that the objection to the Virginia plan does not constitute a decision by the Reagan administration to support extension of the controversial clearance section.

Robb, whose father-in-law, then president Lyndon B. Johnson, made passage of the landmark 1965 legislation a key element in his Great Society program, also said he opposes the extension.

But Robb added "I share the concern" of many black Americans who say "symbolism is involved" in the decision whether to extend the act. Robb said he and Coleman are agreed that "we can protect the legitimate rights" without the clearance section of the act.