The Supreme Court heard arguments yesterday in a free speech dispute over whether a public university can force students to pay "activity fees" that help support campus groups whose political stands they oppose.

The University of Wisconsin is appealing a federal court ruling that struck down its mandatory activity fees, which are channeled to a variety of student organizations, including feminist and environmental groups, as violating the speech rights of students who disagreed with the groups' views. The university contends the fees are a legitimate way to enrich campus debate, and say that students cannot opt out of paying the fees any more than they can refuse to pay tuition for classes whose content they oppose.

In spirited oral arguments yesterday, Susan K. Ullman, an assistant Wisconsin attorney general, emphasized that the activities fees support a wide range of student voices and do not favor particular views or ideologies.

Jordan W. Lorence, appearing on behalf of conservative Christian students who objected to their money going to liberal-leaning groups, countered that the program infringes students' right "not to speak." "It's an obvious issue of compelled speech," he told the justices.

Many state universities require students to pay activity fees, and an eventual ruling in the case could affect campuses nationwide. But over the course of an hour yesterday, the justices explored so many facets of the dispute that it is difficult to predict how they will rule.

Some of the justices were concerned about student money being funneled to groups that have definite political objectives. Others questioned whether any particular group could be favored or promoted if the pool of money presumably was open to a wide array of organizations. And a few focused on the potential pitfalls of the different ways money is distributed, which include through student boards and, on rare occasions, through student-body votes.

Justice Anthony M. Kennedy, for example, observed that a student environmental organization, Wisconsin Public Interest Research Group, received a $49,000 cut of the activity fees as a result of a student referendum vote. He and other justices questioned whether such a vote necessarily rewards a group with a popular views and, if so, whether the funding scheme can still be considered neutral.

"It's just interested in propagating its views," Chief Justice William H. Rehnquist said, challenging Ullman's characterization of WISPIRG as a student "service."

Similarly skeptical about the neutrality of the activities fee program, Justice Antonin Scalia asked whether a student chapter of the Ku Klux Klan would get money. Ullman said yes, if it met the application requirements.

But some of the justices appeared sympathetic to the university's interest in providing a forum for diverse viewpoints. Kennedy noted that since "ancient times" universities have been the scene of great debates.

Scott Southworth and two other students who objected to paying 1995-96 student fees of $331.50 won their case in lower courts. In ruling in their favor, the U.S. Court of Appeals for the 7th Circuit referred to a "right not to speak" and relied on prior Supreme Court cases barring a government union from spending dues on ideological causes and prohibiting a state bar from using dues for political activities.

But the Supreme Court may not rely on the same line of cases. Wisconsin has urged the justices to expand on a 1995 decision in which it ruled that when a university sets up a general policy for disbursing student activity funds, the university must subsidize secular and religious publications on the same basis.

A ruling in the case of Board of Regents of the University of Wisconsin System v. Southworth is expected next spring.