Virginia Democratic senatorial nominee Andrew P. Miller traveled to the District of Columbia yesterday to voice his opposition to the city's proposed commuter tax as a setback not only for the commuters but for the District itself.

At a morning news conference outside Mayor Walter E. Washington's office in the District Building, Miller said the commuter tax "would obviously set back the cause of regional government" at the very time when the District needs the help, interest and cooperation of its surrounding jurisdiction."

"I want to make it clear that I am not unsympathetic to the problems of the District of Columbia," Miller said, adding that he favors giving voting power to the District's representative in the House.

As Virginia's attorney general, he said, he helped resolve the longstanding dispute between the District and Virginia over the city's Lorton Reformatory in Fairfax County.

"In the U.S. Senate, I would certainly work with the District of Columbia to see that appropriate alternatives are worked out (and) there was adequate funding," he said."But by the same token I think this particular measure (which could create the commuter tax) is a step in the wrong direction."

Miller said his opposition to commuter taxes is not simply directed at the one proposed by the District. The state of Virginia, he said, does not permit them, although the city of Richmond has the same sort of inter-jurisdiction problems with surburban commuters as does Washington.

"This is not an easy matter to resolve," Miller said, "and I think everybody wants to be fair." But he said he thought imposition of the commuter tax "would be viewed by the citizens of Northern Viginia as an unfair step" and would thus undermine regional cooperation on such important projects as full funding for the complete 100-mile Metro system.

Last week a House District sub-committee approved a bill that would amend the District's home rule charter to enable the City Council to impose the commuter tax. The full House District Committee is scheduled to consider the measure today.

The commuter tax proposal, long a subject of controversy, is seen by its backers in the District of Columbia as vital to the financial health of the city.

But it has also been attacked consistently as unfair by legislators whose constituents in the Virginia and Maryland suburbs would have to pay the levy.

The commuter tax would tax non-residents who work in the city at 1/3 the rate imposed on D.C. residents and give those same commuters a dollar-for-dollar credit against their state income taxes in Virginia and Maryland.