WASHINGTON - When it comes to the rights of the detainees at Guantanamo Bay, the Supreme Court, and not the president or Congress, will have the last word.

That was the clear part of the message to emerge yesterday from the Supreme Court argument on whether the men held as enemy combatants at the U.S. naval base in Cuba have been given constitutionally adequate means to challenge the legality of their detention.

A majority of the court appeared ready to agree that the detainees were entitled to invoke some measure of constitutional protection.

These justices seemed to treat that threshold issue as a bridge they had crossed long before they entered the courtroom.

That was the easy part of what the detainees' lawyer, Seth P. Waxman, called "a particularly easy, straightforward case."

Less clear was what happens next.

The federal appeals court here ruled in February, in the decision the justices are reviewing, that because the detainees had no constitutional rights in the first place, no problem was posed by a 2006 law that stripped the federal courts of jurisdiction to hear habeas corpus petitions filed by detainees seeking to enforce those rights.

So the appeals court did not rule on, or even examine in any detail, the procedures Congress set up to provide the detainees with limited rights to challenge their designation as enemy combatants.

Solicitor General Paul D. Clement, urging the court to uphold that ruling, argued that procedures provided by the Detainee Treatment Act of 2005 and the Military Commissions Act of 2006 were better than adequate. The detainees were getting more access to judicial review, he maintained, than the writ of habeas corpus would have given foreign combatants when the Constitution was adopted.

By the end of the argument, a session that lasted 23 minutes beyond its allotted hour, it seemed most likely that the court would draw a road map for the appeals court to follow in expanding the procedural protections available to the detainees.

The significance of the eventual ruling, due by early summer, will lie in what features the road map contains.

And that, in turn, may depend on how far Justice Anthony M. Kennedy is willing to go in joining an opinion that will in all likelihood be joined by justices John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer. Kennedy, presumed to hold the balance in this case, was the focus of much attention by both sides.

His few questions suggested that his main interest was in how far the appeals court might be able to go to remedy any flaws in the existing procedures.

_ Meanwhile yesterday, the Associated Press reported, a military judge at Guantanamo rejected a request by lawyers for a detainee to talk to the alleged mastermind of the Sept. 11 attacks.

Lawyers had hoped that Khalid Sheikh Mohammed could establish that their client, Salim Ahmed Hamdan, a former driver for Osama bin Laden, was not a hard-core terrorist. *