A federal jury began deliberations today in the political corruption case against Baltimore's chief prosecutor, William A. Swisher, who is charged with halting a major criminal investigation in return for election support from a powerful political boss.

Federal prosecutors have charged that the boss, the late James H. (Jack) Pollack, "bought himself a state's attorney" by pumping money and support into Swisher's 1974 campaign that propelled him from the obscurity of a neighborhood law practice to head the largest prosecutor's office in the state.

Swisher's lawyers have asserted that Pollack was just one of several Swisher supporters and that there was nothing sinister or evil in their relationship. The probe Swisher is alleged to have halted at Pollack's behest actually "died a natural death" in 1975, the defense lawyers said.

The U.S. District Court jury deliberated for four hours late today and is scheduled to resume Wednesday morning.

The case is the latest in a series of federal corruption prosecutions of Maryland politicians, including former governor Marvin Mandel, former vice president Spiro T. Agnew and several lesser officials.

In his closing argument to the jurors today, Assistant U.S. Attorney Daniel Hurson asked: "Who will watch the watchman? Who will protect us from those whose duty and charge it is to enforce the laws?" And he replied, "You will."

With that and two hours of instructions on legal points from Judge Alexander Harvey II, the jurors retired to deliberate on the 18 counts of mail fraud and three of extortion against the 46-year-old prosecutor.

The government called more than 70 witnesses in laying out a three-pronged case of circumstantial evidence against Swisher. It charged:

That Pollack plucked Swisher from obscurity and personally directed his 174 campaign, collecting campaign funds to ensure his victory.

That, one elected, Swisher surrendered hiring decisions to Pollack, who often received payoffs from those who got jobs as investigators and prosecutors.

That Swisher abruptly halted a major investigation of a city bid-rigging scheme after the suspects paid Pollack about $25,000.

The prosecutors claimed Swisher received nearly $4,000 of the payoff, although no testimony linked him directly to any money paid to Pollack.

According to the testimony, Swisher made a cash deposit of $3,920 into his bank account in September 1976, more than a year after the city corruption probe was halted. But last year, when federal . . . agents questioned him, Swisher was unable to recall where the money came from, according to testimony. However, when Swisher took the stand last month, he said he was "a saver," and that the money was part of a "cash kitty" he had built up.

In his closing argument to jurors, defense counsel Aubrey M. Daniel III accused prosecutors of "stretching (the facts) since the day they walked into this courtroom" and of using "every conceivable innuendo" to try to prove Swisher guilty.

Swisher acknowledged he had hired people recommended by Pollack, but insisted that was routine political patronage and not a crime. He emphatically denied ever talking to Pollack about the sensitive city corruption investigation that is the centerpiece of the prosecution's case.

"Jack (Pollack) supported me," Swisher said of his 1974 election campaign. "I appreciated it. But he did not own me, period."