H.R. Haldeman, whose storied loyalty to Richard M. Nixon shattered in the backlash of Watergate, will be paroled from prison Dec. 20 after serving 18 months of a four-year sentence for obstruction of justice and perjury.

The U.S. Parole Commission set the release date yesterday, acting on the recommendation of examiners who interviewed Haldeman at the Lompoc, Calif., minimum security prison last month.

The commission had the option of paroling the former White House chief of staff any time after he had served the minimum time since by all reports he has been a model prisoner.

Haldeman entered Lompoc June 21 last year after the Supreme Court refused to accept the appeal of his Watergate conviction. In a petition later, he said imprisonment made him "useless to the world, a burden on society . . . living a totally wasted life at this time."

As chief of staff to Nixon in his first term, and the president's closest confidant, Haldeman wielded enormous power. But their friendship turned sour when Nixon refused to pardon Haldeman as a final presidential act and when he later alluded to Haldeman's guilt.

In his book, "The Ends of Power," Haldeman made the final break with the former president, characterizing Nixon as being behind the Watergate cover-up "from Day One."

Haldeman, now 51, was convicted after a three-month trial of conspiring to obstruct justice, obstructing justice and three counts of perjury. His co-defendants, John D. Ehrlichman and John N. Mitchell, were convicted of similar charges.

All were sentenced by U.S. District Court Judge John J. Sirica to serve 2 1/2 to 8 years in prison. After hearing taped statements of contrition, Sirica cut the terms to one-to-four.

Ehrlichman, the former Nixon domestic counselor who was convicted both in the Watergate cover-up and Ellsberg break-in cases, was released from the federal prison camp at Safford, Ariz., April 27 after also serving 18 months.

Mitchell began his imprisonment at the Maxwell, Ala., Air Force Base a day after Haldeman surrendered at Lompoc. But the former attorney general missed his preliminary parole interview because he was free at the time on a medical furlough that kept him out of prison for five months. The interview is scheduled for early July.

In prison, Haldeman has worked seven-hour days, seven days a week as a lab chemist in the sewage processing plant.