WHAT IS AN EDITORIAL writer supposed to make of the way Andrew P. Miller, the Democrat, and Richard D. Obenshain, the Republican - each a product of his party's convention - are approaching their contest for a U.S. Senate seat from Virginia? Usually at this stage of an election campaign, we can safely pull out Political Editorial Comment No. 1-A, which is the one that raps knuckles all around while bemoaning an excess of campaign hot air and an absence of substance. It is often embellished by Aside No. 4-C, the one pointing to an overemphasis on public-relations gimmickry and "charisma" as a shallow substitute for a discussion of the issues. But so far, the standard stuff of campaign shain, neither of whom has ever been indicted for possession of charisma, are actually talking about real issues.

And in some instances, the two candidares are defining significant differences of opinion.

For example, Mr. Obenshain says he would allow states to rescind ratification of the Equal Rights Amendment, which he opposes, while Mr. Miller, who favors the amendment, claims that such action would be unconstitutional. Mr. Obenshain attacks unions and supports a nationl law banning union-shop agreements; Mr. Miller says unions are "part of American society" while expressing opposition to a series of national bills supported by labor. Mr. Obenshain also is calling for a 33 percent across-the-board cut of federal income taxes over three years. Mr. Miller has rejected the proposal, arguing that it would create an enormous federal deficit.And of interest to Northern Virginians, not to mention anybody else in this metropolitan region, both candidates support completion of the full 100-mile Metrorail system with heavy federal financing.

At least one Virginia political tradition seems intact: The candidate of the party in the White House usually doesn't make too much of it (Henry Howell did, and he lost). Mr. Obensham tries to link Mr. Miller with President Carter, but Mr. Miller responds that he wouldn't be a White House yes-man (and after Mr. Carter's campaigning for Mr. Howell, and against Mr. Miller, in the last Democratic primary, Mr. Miller certainly doesn't owe the White House much more than that). Still, on the state level there is another difference this year. For now at least, each candidate for governor is enjoying unusually wide support within his party while appealing for support from members of his opponent's party. When you couple all of this with the eagerness of both men to meet in debates, what you wind up with is the promise of a spirited and stimulating political contest in the months ahead.