The jury in the espionage trial of Ronald W. Pelton began delibrations today after hearing an impassioned closing argument form prosecutors who branded Pelton a manipulative con artist who sold out hid country, and a plea from the defense to judge Pelton not on his ''moral guilt'' but on his legal guilt.

''The man is a spy,'' prosecutor John G. Douglass told the jury in a steely whisper this afternoon. Douglass, pointing accusingly at Pelton, said: ''My client is th is nation . . . and I'm here to tell you that that man is not going to walk into this court and trash the interests of that client the way he trashed the top secret, classified, sensitive national defense projects for a period of five years before he got caught and stopped.''

Another prosecutor, Robert McDonald, read to the jury the security oaths Pelton signed when he joined the National Security Agency in 1965: ''I solemnly swear not to reveal classified information to anyone . . . .''

Pelton, he said, once a trusted employe of one of the government's most secret agencies, ''broke and betrayed'' that trust when he sold Soviet agents information about five top-secret NSA intelligence-gathering systems ''aimed at intercepting and exploitingunscrambling'' Soviet communications.

Defense attorney Fred Warren Bennett, arguing that FBI agents had violated Pelton's constitutional rights before they arrested him in November, implored the jury to disregard the incrimminating statememts the agents extracted from his client and to set aside any ''personal dislike'' for Pelton and the crimes he is alleged to have committed.''It 's your duty to find legal guilt,'' he told them, ''not moral guilt.''

Pelton's statment to the FBI was ''irrelevant if it was not made freely, Bennett told the jurors.

U.S. District Judge Herbert F. Murray, in two hours of instructions to the jury, said it would be up to the seven men and five women to determine whether Pelton's statements to FBI agents had been made voluntarily.

He said they should consider seven legal criteria, including whether the agents tricked or coerced Pelton or made any implied promisesto him. If the jury finds Pelton's statementthe centerpiece of the government's casewas not voluntary, it must be disregarded, Murray said.

After several hours of deliberation, the jury requested clarification of Murray's instructions on whether the confession was voluntary.

Murray also took the unusual step of correcting Douglass for asserting that he represented ''the nation'' as prosecutor. ''While that may be figuratively true,'' Murray told the jurors, ''it may well be more accurate to say that he represents the United States government.''

The jury, which has been sequestered since the trial began nine days ago, retired for the night at 11 p.m. after deliberating for 6 1/2 hours. It is scheduled to resume deliberations Thursday morning.

Pelton, 44, is charged with three counts of espionage, one count of conspiracy to commit espionage and one count of unauthorized disclosure of classified information about U.S. communications intelligence. The last count carries a maximum penalty of 10 years in prison. Pelton faces a possible life term for each of other four charges.

Pelton, an NSA employe for 14 years before he left the agency in 1979, is accused of selling information to the Soviets between 1980 and 1985 in return for $35,000.

Pelton filed for bankruptcy six months after leaving his $24,500-a-year job at NSA, and is alleged to have contacted Soviet agents first in January 1980.

According to a former supervisor at NSA who testified for the government in the trial, Pelton had broad knowledge of and access to enciphered Soviet communications.

Prosecutors said Pelton's skill as a budget negotiator for his NSA department and his experience later as a salesman amply prepared him for his interview with FBI agents.

''This is not some 16-year-old getting beat on with a rubber hose.'' said Douglass. ''He bluffed, he probed, he gave up a little bit at a time until he hung himself.''

''He was a con artist, a wheelerdealer,'' Douglass said to the jury. ''He's trying to con you.''

Douglass said the FBI agents conducted their interrogation of Pelton ''by the book.'' Agent David Faulkner, who ran the interview, ''ought to get a medal, ought to get a commendation,'' Douglass said. ''He got the spy to confess.''

But Bennett told the jury that Pelton was ''working against a stacked deck'' in his interview with the FBI agents.

Pelton was led to believe the FBI wanted to know what he sold to the Soviets for counterintelligence purposes, not so they could use his admissions for prosecution, Bennett said.

''They told him, ''You can talk to us. We have it from other sources. What you tell us will not be used against you,' '' Bennett said ''Is it being used against him?'' he asked the jury.

Douglass stressed that Pelton ''gave up information which he himself admitted on the witness stand could potentially endanger lives.''

Douglass was alluding to Project A, the top-secret NSA program that intercepted communications link.''

Informed sources have told the Washington Post that Project A was located in the Sea of Okhotsk near the eastern Soviet mainland.

Pelton was accused of walking into the Soviet Embassy in Washington in January 1980 and pointing out the location of Project A on a map.

Under FBI interrogation last November, Pelton circled a spot on a map to duplicate what he had disclosed to the Soviets.

''Mr. Bennett talks about the fact that he pointed out the wrong location to the agents for Project A. It was several hundred miles off,'' said Douglass.

''It didn't matter. It would be more than enough to let the Soviets know where the communications link was. They know their own communications links like you know where your phone is,'' the prosecutor declared.