Thomas Penfield Jackson was a 38-year-old Washington lawyer when he took his first public position on a privacy issue.

Speaking for the 4,500-member D.C. Bar Association, he opposed a 1975 proposal to make public the names of Washington lawyers under investigation by a disciplinary board for alleged misconduct. His reasoning: "media coverage is more efficient here" and news organizations are likely to publicize the allegations.

Twelve years later, Jackson, a U.S. District Court judge, collided with the Washington press corps over a similar issue: his decision to conduct interviews in private with potential jurors in the perjury trial of former White House aide Michael K. Deaver.

News organizations objected to the practice, and a federal appeals court panel sided with the press, a decision that so shook the jurist that he yesterday ended the criminal trial and reset the case for October.

To Jackson's friends and some of his critics it was an action typical of a cautious, conservative judge who is said to worry greatly about whether his opinions are overturned or upheld by appeals courts.

In the five years since President Reagan named Jackson to the U.S. District Court bench, the Washington native has seen many of his rulings reversed.

The appeals court ruling that his interviewing technique violated Supreme Court standards may have shaken him because, some friends said yesterday, Jackson prides himself on being well-prepared for the cases he hears and for knowing courtroom procedure. A number of other judges in Washington, among them some of the court's better-known liberals, also have selected jurors in other cases after private questioning, several lawyers said.

Those cases, however, have not attracted the attention that the Deaver trial commands. Until now most of Jackson's major cases have involved civil disputes, the type of cases he specialized in as a lawyer.

"He was a lawyer's lawyer . . . one of the best trial lawyers in this town," said Patricia Gurne, a lawyer at the firm of Jackson & Campbell, which was founded by the judge's father.

James Bierbower, who was president of the D.C. Bar when Jackson was nominated by Reagan in 1982, said that Jackson was a tough competitor in the courtroom and remains so as a judge. "He is a no-nonsense judge, a very careful judge, always well-prepared and always firm," Bierbower said.

"He's a very conservative judge for all that implies," said Lois Williams, director of litigation for the National Treasury Employees Union, a group that frequently sues the government over personnel issues. The judge has tended to side with the government on the issues, most notably upholding its authority to order reductions in force.

His best-known decision, rejecting a government effort to force a recall of General Motors Corp. X-car models as unsafe, was made after years of litigation and is on appeal.

But in a major government case, Jackson parted with the Reagan administration last year over a claim that the president had authority to defer approved spending from one year to the next. Jackson ordered the release of $5 billion in housing funds, a decision that was upheld by an appeals court.

As a Washington lawyer, Jackson, a Dartmouth College and Harvard Law School graduate, was best known for his work as attorney for the Committee for the Reelection of the President, the group that financed President Richard M. Nixon's 1972 campaign. Jackson, a Republican precinct official in Montgomery County, defended the committee from lawsuits that grew out of the campaign.

After Reagan's election in 1980, Jackson was mentioned as a candidate for the D.C. Superior Court and D.C. Court of Appeals, but a local nominating panel did not endorse him because, according to some lawyers, he belonged to the Chevy Chase Club, which had no black members.

Today that membership is the first of three memberships listed by the judge in his official biography. The others are the Metropolitan Club and Capitol Hill Club.