IT MAY BE all right on a Chinese restaurant menu, but it's not on Maryland's Democratic presidential ballot: requiring people to select one item from Column A and one from Column B. That's the ruling of federal Judge Walter E. Black Jr. in a case brought by Nicholas Bachur Sr. against the state of Maryland and the state and national Democratic parties. Mr. Bachur is a registered Democrat in the 3rd congressional district who was infuriated to learn in May 1984 that Democratic party rules, which state law accepts as governing, required him to cast no more than four of his votes for men and four for women in choosing the 3rd district's eight national convention delegates. He filed suit and persevered after the 1984 election. Judge Black heard oral argument in June 1985 and handed down his 58-page opinion the other day.

Maryland state officials and Democrats argued that they were just trying to implement the national party's requirement that half the national convention delegates be women. They had on their side Supreme Court rulings that political party rules should prevail over state laws. But constitutional rights prevail over state laws and over party rules, and Judge Black ruled that Mr. Bachur's constitutional right to vote for the delegates of his choice was infringed and that this rule was invalid. Any court these days, he said, would overturn a rule that limited the number of black candidates who could be voted for in a multicandidate election, so a rule that limits the number of men and women Maryland Democrats can choose must fail too. A silly law, we said when Mr. Bachur filed suit and, the decision persuades us, an unconstitutional one as well.

But there may be more to this case than just the overturning of an egregiously silly party rule. It could strike at the Democrats' whole set of rules, which amount to quotas for blacks, Native Americans, Asian/Pacifics, Hispanics and youth, as well as women. Judge Black points out that there are plenty of ways to get 50 percent women in your delegation if you don't hold a primary. But most states do. And if states can't bar you from voting for as many men or women as you want, can states cooperate with the elaborate scheme of the Democratic Party that is designed to keep you from voting for as many white, non-Native American, Anglo, Occidental/Atlantic, middle-aged delegates as you want? Implicitly the judge rejects the assumption that seems to underlie the Democrats' rules: that only women can represent women, only Asian/Pacifics can represent Asian/Pacifics, etc. Instead he is saying: let the voters who can represent them best decide.

In practice most Democratic candidates will put up ethnically, sexually and chronologically balanced slates that will be proof against challenge under the party's rules. But if Judge Black's decision is upheld by higher courts or followed elsewhere, the whole elaborate structure of the Democrats' distributional requirements could come under attack from a disgruntled candidate with little to lose and at the most embarrassing of times.