The morning was all his, and Oliver L. North knew it. So, it seems, did the country. Even as North continued his strong self-defense of his Iran-contra role, a room in the Senate Russell Office Building was filling with flowers sent him from across the nation, and citizen supporters were literally offering checks for his defense to Capitol guards.

Inside the hearing room, North expounded jauntily on foreign policy, cited constitutional scholars to support his belief that his actions had been legitimate and lectured Congress on the unfairness of its investigation -- all in the same sincere, husky tones that have become familiar to millions this week.

Then it changed. As the morning was North's, the afternoon belonged to Arthur Liman, the litigator from New York who is chief counsel of the Senate select committee. After three hours answering Liman's questions, North's demeanor was transformed. The bravado had become hesitancy; the assured Marine suddenly seemed very alone.

Liman's earlier withering, caustic cross-examination of retired major general Richard V. Secord stamped him as the classic tough-guy prosecutor type. But it was a different Liman who greeted North after the lunch recess yesterday afternoon. Instead of the relentless, confrontational questioner, this Liman seemed more concerned about North's problems, and his initial question was delivered softly.

Last Nov. 25 must have been "one of the worst days in your life," he said solemnly, gazing straight at North sitting at the witness stand before him.

That was the day North was fired by the president for his part in diverting Iranian arms sale profits to the Nicaraguan contra forces -- and, as Liman quickly established, the day that Oliver North's life changed forever.

Liman proceeded to reveal the isolation of North, the brave Marine who willingly said he was prepared to "take the rap" for the Iran-contra diversion scandal, but never imagined that part of his fall might mean going to jail on criminal charges.

"I would be the person who would be dismissed or reassigned or fired or blamed or fingered or whatever one wants to use as a description," North explained yesterday. That, in his mind, was part of a plan; part of the core of being a covert operator who understood the critical importance of "plausible deniability" for the administration.

North never anticipated, and was shocked to discover, that he was also being fitted for another role as the principal target of a totally unexpected criminal investigation of the unraveling operation.

He was, as he acknowledged to Liman, "prepared to take the rap for political purposes, but not for criminal purposes."

This was just the answer Liman expected, or so it seemed from his line of questioning. Liman looked as much like a student of his witness' personality as the hard-nosed litigator. He did not break North as a witness yesterday afternoon on Capitol Hill, but he did show that North was -- and felt that he was -- a man betrayed, a man whose "mind set changed considerably" when he realized he had been left alone by his superiors. His attitude changed profoundly, he testified, from willingness to accept responsibility in public for what had happened to battling to protect himself.

As the afternoon session began, the Senate Caucus Room was taut with anticipation of a confrontation between gladiators. Liman preserved the tension, but seemed to surprise North by repeatedly indicating sympathy for his plight and respect for his military record, his valor and his values. Liman, it seemed, had come for a psychological duel, not a shoot-out.

In the end Liman seemed to adopt North's view of his situation. "Now," he said at one point, "it is correct, sir, that you were put in a position in which everybody who's eager and content to have Ollie North do whatever was necessary . . . as long as you didn't create a record pinning it on them? . . . You do it, you provide the deniable link, you take the rap, if it gets exposed. That was what this was all about, right?"

"I have testified to that," North agreed.

The key was Nov. 25 -- the starting point for Liman's interrogation. As North's answers made clear, the former White House national security aide had long accepted the role -- assigned him, he said, by the late CIA Director William J. Casey -- of scapegoat in the Iran-contra affair. At one point yesterday North referred to the role as that of a Roman centurion.

But North's willingness to play that role changed, Liman's cross-examination established yesterday, when North watched Attorney General Edwin Meese III's news conference Nov. 25. Meese announced that North had been fired, and that a criminal investigation would be launched into the Iran-contra diversion.

"There was probably not another person on the Planet Earth as shocked as I was to hear that someone thought it was criminal," North testified. "And I can tell you that that shock was compounded when I heard later that there was to be an independent counsel, and further compounded when I was the only name in the appointment order for that independent counsel -- the only person on the Planet Earth named in that appointment order, counsel."

As the afternoon progressed and the newly subdued North appeared uncharacteristically hestitant at times, his attorney, Brendan V. Sullivan Jr., was moved to angry outbursts in repeated, unsuccessful efforts to redirect the line of questioning.

But the focus remained on Liman's portrait of Oliver North, the good soldier who had been given a mission to shield the blame for others -- and in effect had been set up.

The fall he took was not one he anticipated, Liman kept suggesting. Nor was it in keeping with North's own values about honor and trustworthiness inculcated from his Naval Academy days to the fields of combat in Vietnam.

By the end of the afternoon, Liman had drawn from North repeated acknowledgments that in carrying out his superiors' wishes he had repeatedly violated those values. He had to claim in the end that, yes, he did lie and deceive, but in a good cause, and on orders; yes, he did proceed on the legal theory that a White House aide was not restricted by congressional bans on aid to the contras because, he argued, the Constitution gave his president free reign.

And, he acknowledged to Liman, he now knew he had been left alone: "I do honestly believe that they expected that Ollie would go quietly. And Ollie intended to do so right up until the day that somebody decided to start a criminal prosecution."