To the public, Secret Service agents are forbidding-looking men and women with dark glasses covering their eyes and dark suits covering their guns, always standing near the president.

But to a president, they are so familiar and trusted they seem almost a part of the furniture. So trusted, in fact, that presidents have felt completely at ease talking about anything or doing almost anything in front of agents, always confident their secrets would be kept.Until now.

Secret Service officers are being asked questions from independent counsel Kenneth Starr about things they've seen or heard while protecting President Clinton. The precedent could dramatically change the way presidents look at the agents around them and how the chief executives do their job.

It is almost inevitable that a president would see the agents standing nearby as potential snitches, now vulnerable to subpoena from a court or congressional committee.

It would be forever on the mind of presidents as they go through their day, whether they are pondering something illegal, something embarrassing like an extramarital affair or simply the kind of blunt and candid talk about political strategy or foreign relations that is essential to the job but would be embarrassing if made public.

The men who have held the office are split.

Clinton said it could have a "chilling effect" on his relationship with the agents and the way he works in front of them.

Former President George Bush similarly said that "if a president feels that Secret Service agents can be called to testify about what they might have seen or heard, then it is likely the president will be uncomfortable having the agents nearby."

But former Presidents Jimmy Carter and Gerald Ford said the agents should testify.

Other White House veterans and presidential scholars think presidents will have two choices in how they will adapt - asking the agents to step away whenever they want to talk or act privately, or limiting those private moments to places like the Oval Office, where they often work without agents in the room.

Of course, presidents also might think twice about doing something illegal or immoral. But few think presidents will stop having frank discussions about national politics or policy.

"Presidents could adapt," said George Edwards, a presidential scholar and political scientist at Texas A&M University.

"The president will still have to have these conversations. I would feel better if the Secret Service did not have to testify. But the republic is not in peril because of this."

Maybe not the republic. But some believe that the men or women who occupy the office could put themselves in peril if they are compelled to talk about something or do something and tell agents to step farther away so they cannot see or hear.

"Without a doubt, there is going to be a separation between the president and the Secret Service agents," said former agent Tim McCarthy, who was shot while protecting President Ronald Reagan during an assassination attempt in 1981.

Standing close to the president whenever he is out in public, and sometimes in private, McCarthy said during a recent appearance on CNN, "You hear an awful lot. You see an awful lot." Presidents will be aware that a grand jury could ask an agent almost any question about presidential behavior, he said.