WASHINGTON (AP) _ The Bush administration Wednesday defended undercover sting operations, telling the Supreme Court that the government properly targeted a 61-year-old Nebraska farmer convicted of receiving child pornography.

Government lawyer Paul Larkin maintained that it was not illegal entrapment to mail the farmer, Keith Jacobson, ''kiddie porn'' solicitations for 28 months until he ordered a magazine.

But Jacobson's lawyer, George Moyer, said the government had violated his client's rights.

Moyer said, ''It was crystal clear to the government'' after more than two years of mailed solicitations that Jacobson was not part of a ''child pornography underground'' that exploited or abused children. ''At that point, he should have been left alone.''

The justices must decide whether the government's conduct by definition was illegal entrapment, and therefore the question of Jacobson's guilt or innocence should not have been submitted to the jury. A ruling is expected by July.

Jacobson attended the argument session. Afterward he stood before television cameras and reporters outside the court building and said the government had abused its power.

''I didn't know what was happening to me. I sort of got sucked in,'' he said. ''If I had been left alone, I wouldn't have bought'' the pornographic magazine.

The gray-haired Jacobson, who wore an American flag pin on his tweed jacket, said he is optimistic about the outcome of the case. ''I feel positive about it,'' he said. ''I figure it's in God's hands and the justices' hands.''

Some of the justices vigorously challenged Jacobson's lawyer during the one-hour argument session.

Justice Sandra Day O'Connor said Moyer's attack on government stings could threaten other undercover law enforcement operations - such as the use of pawn shops to catch people who steal property or purchase stolen goods.

''If you're correct,'' she told Moyer, ''then some of these pawn shops are operating illegally.''

Also at issue, she suggested, could be sting operations in which government officials are offered bribes. The high court has permitted such investigations.

Justice Antonin Scalia said the government may have had adequate evidence that Jacobson was predisposed to buy child pornography and therefore he was not entrapped by the repeated mailed solicitations.

Police found Jacobson's name on a San Diego, Calif., pornography bookstore's mailing list in 1984. He had ordered two nudist magazines from the store that were legal to purchase.

Jacobson, of Newman Grove, Neb., was convicted of receiving in 1987 a copy of a magazine called ''Boys Who Love Boys.'' It was described in a catalog as showing ''11-year-old and 14-year-old boys get it on in every way possible.''

Jacobson was sentenced to two years probation and 250 hours of community service. He lost his job as a school bus driver the morning after he was charged and said he was ''humiliated and depressed'' for months.

Moyer told the high court, ''I don't disagree that child pornography can be pernicious.''

But he said the basis for the government's investigation of his client was Jacobson's purchase of a legal nudist magazine.

''You can't assume someone will engage in criminal activity because he engaged in it when it was legal,'' Moyer said.

Larkin said the government needs wide latitude to investigate child pornography, which he said is engaged in ''enticing, seducing and molesting children.''

Distributors of child pornography are ''very clandestine,'' he said. Those who buy and sell such material are ''cagy'' and it may take years to catch them, he said.

''The government can't lean on you to force you to commit a crime,'' Larkin said. But he said in Jacobson's case there was ample evidence for a jury to decide he was predisposed to receiving illegal pornography.

''At no time did he call the police or throw these things away,'' he said.

Only Justices Harry A. Blackmun and Clarence Thomas did not question the lawyers.

During Thomas' recent confirmation hearings, a former employee told the Senate Judiciary Committee that Thomas sexually harassed her by discussing pornographic movies he had seen. Thomas denied the allegations.

The case is Jacobson vs. U.S., 90-1124.