THE EROSION of federal habeas corpus protection takes place one flawed Supreme Court decision at a time. Each may be negligible by itself. Yet collectively these decisions -- the Supreme Court handed down the most recent Monday in O'Sullivan v. Boerckel -- have created a set of rules under which a person wrongly convicted in a constitutionally flawed trial faces far too high a hurdle in getting that conviction reviewed.

The latest case involves a prisoner in Illinois named Darren Boerckel, convicted in 1977 for the rape and battery of an 87-year-old woman and sentenced to a lengthy term. The Illinois court of appeals rejected his claims. When he raised three of them before the Illinois Supreme Court, it declined to consider them. Years later, he filed a federal habeas corpus petition seeking to get these three and the other issues reviewed by the federal courts. The Supreme Court, however, has now ruled that because he had earlier petitioned the Illinois Supreme Court on only three matters, he was eligible for federal review only for those three. On the others, the court held, he had waived any right to habeas review by failing to exhaust state court appeals. The theory is that the federal courts should not intervene in a state court matter that the state courts have not had a full chance to address.

The court so held even though the Illinois Supreme Court exercises what is called discretionary review, a system under which some appellate courts do not hear every case put before them but choose. The Illinois court rules actually seem to discourage litigants from appealing most cases to their Supreme Court. In other words, in order to hold that Mr. Boerckel had waived his habeas claims, the U.S. Supreme Court had to rule that he should have clogged the docket of a state court with matters it would be unlikely to consider.

We do not know whether Mr. Boerckel's habeas case has merit, but that is not the point. One wouldn't want an innocent person whose trial was constitutionally defective to be barred access to federal court review on grounds so far removed from the substance of his claims as the ones the high court has used to limit review in this case.