WASHINGTON - A Supreme Court justice has hinted strongly to Congress that a key case will be decided in favor of keeping federal courts open for the growing volume of civil rights damage claims. The highly unusual hint came March 9 in testimony by the newest justice, Sandra Day O'Connor, before the House Appropriations Subcommittee that is studying the court's $14.9 million budget request. The issue arises in a pending Florida case that has the potential of cutting off many civil rights cases. She did not say, in so many words, how the court would rule. But she did urge Congress to pass a law to achieve that result by requiring most civil rights cases to be pursued first with state agencies, instead of going directly to the federal courthouse. It would not be necessary for Congress to act, of course, if the justices were to interpret present law to give state agencies priority in handling such cases. Asked after the hearing if the issue she had discussed were not the same one now under review by the court in the Florida case, O'Connor replied: "I will rest on what I said." Just two weeks ago, the court heard lawyers argue the case. Under normal procedures, the justices would have cast their preliminary vote on the case at their secret conference on Friday. O'Connor's promotion of a federal law to shunt more civil rights cases to state agencies echoed a proposal she made in a law review article last summer, before she was chosen for the Supreme Court. An Arizona appeals court