President Obama’s proposal for a new legal system in which terrorism suspects could be held in “prolonged detention” inside the United States without trial would be a departure from the way this country sees itself, as a place where people in the grip of the government either face criminal charges or walk free.

There are, to be sure, already some legal tools that allow for the detention of those who pose danger: quarantine laws as well as court precedents permitting the confinement of sexual predators and the dangerous mentally ill. Every day in America, people are denied bail and locked up because they are found to be a hazard to their communities, though they have yet to be convicted of anything.

Still, the concept of preventive detention is at the very boundary of American law, and legal experts say any new plan for the imprisonment of terrorism suspects without trial would seem inevitably bound for the Supreme Court.

Mr. Obama has so far provided few details of his proposed system beyond saying it would be subject to oversight by Congress and the courts. Whether it would be constitutional, several of the legal experts said in interviews, would most likely depend on the fairness of any such review procedures.