William A. Swisher, Baltimore's chief prosecutor, took the witness stand at his trial on political corruption charges today and denied he ever struck a deal with the city's late political boss James H. (Jack) Pollack.

Swisher also denied taking bribes from anyone and said he rebuffed Pollack on several occasions when Pollack talked to him about patronage jobs.

The trial, in its seventh week in U.S. District Court here, centers on allegatins that Swisher stopped a sensitive investigation of city demolition contractors in exchange for Pollack's powerful political and financial support during Swisher's underdog campaign for state's attorney in 1974.

Swisher is charged with 18 counts of mail fraud and three of extortion. Six counts of income tax evasion and filing false returns in connection with the scheme will be tried separately.

Swisher's defense attorney Aubrey M. Daniel III asked him today whether Pollack owned him "lock, stock and barrel" because of pollack's assistance in the campaign.

"No, he did not," Swisher replied in a sometimes trembling, nervous voice. "Jack supported me, I appreciated it, but he did not own me, period."

Swisher said he was driving Pollack to a "political thing" one day during the campaign and Pollack "pulled something out of his pocket, a piece of paper. He said, 'I want you to look at this and sign it. I want you to give me some jobs or patronage if you are elected.'

"I said, 'No, Jack, I'm not signing anything like that.' He said, 'Why not"? Swisher continued. "I've got them from senators and congressmen.'

"You're helping me," Swisher said he told Pollack. "'I'll help you. But I'm not signing anything up front."

"Did you ever make a commitment to him?" Daniel asked.

"No I didn't," Swisher said.

As for patronage, Swisher said he believed in it, and "I practice it. It's the American way. There's nothing wrong with that. There are pros and cons to the patronage system and pros and cons to the civil service system."

Prosecutors said Pollack selected and promoted Swisher to run against Milton B. Allen for state's attorney in 1974. Swisher said today he ran at the request of George Hofferbert, an East Baltimore political boss who had supported Swisher in his efforts to get into politics years earlier.

It was Hofferbert who also suggested Swisher seek Pollack's support, Swisher said. He added that he never kept it a secret that Pollack was helping his campaign because he needed "all the endorsements I could get."

"Did you ever take a bribe from Jack Pollack?" Daniel asked, referring to a bank deposit of $3,920 that the prosecutors said was part of the $25,000 bribe contractors paid to Pollack to stop the investigation into a bid-rig-ging scheme.

"That was my money," said Swisher. "I've always been a saver. There was no way I would take a bribe."