The chief witness in the Senate Ethtics Committee hearing on Sen. Herman E. Talmadge testified yesterday that he repeatedly misled Justice Department investigators looking into allegations of financial wrongdoing by the Georgia Democrat.

Daniel Minchew told the Senate Select Committee on Ethics that he met with Justice Department attorneys John Kotelly and Kathleen Voelker last July. Minchew said he lied to the attorneys because he believed he was still part of a cover-up he said had been ordered by Talmadge.

"I was really very evasive," Minchew said. "I tried to mislead Mr. Kotelly. I tried to give him as little information as possible."

Minchew, who served as Talmadge's chief aide from 1971 through 1974, told the ethics panel that last June he sought an early morning private meeting with Talmadge because he felt his former boss' judgment was slipping.

The 39-year-old witness said he felt Talmadge made "a serious error in judgment" when he told a newspaper reporter that he paid his day-to-day bills out of $5 and $10 contributions thrust on him by supporters.

"There were a number of complicating factors in Sen. Talmadge's life that were well known to myself and other close associates," Minchew said.

"I did not know what his judgment faculties were in 1978," he said.

Talmadge was treated earlier this year for alcoholism. His former wife, Betty, divorced him in a bitter proceeding in 1977.

Recalling their meeting last June, Minchew said he feared Talmadge would reveal a secret scheme that Minchew has said they developed to siphon off $39,000 through a hidden bank account from improper Senate reimbursements and mostly unreported campaign contributions.

According to Minchew he gave Talmadge a memo at the June 14 dawn meeting in Talmadge's apartment library.The memo, which has become a key piece of evidence in the long-running hearing, refers to the diversion scheme. Talmadge has denied knowing about the diversions or the secret bank account.

Minchew said he was instructed by Talmadge to keep the scheme a secret between the two of them. Despite his misgivings about Talmadge's judgment. Minchew said he was going along with the cover-up when he lied to the Justice Department investigators the following month.

Talmadge claims the meeting and the memo from Minchew were part of a blackmail scheme against him by his former aide. He turned the memo over to the Justice Department shortly after the June 14 meeting.

The tangled case has so far come down to a question of the truthfulness of Minchew's testimony. Talmadge's attorneys have sought at every chance to point out contradictions in Minchew's testimony or past lies by him.

In his testimony yesterday, Minchew acknowledged that he did not tell the truth about the size of payments he made from the secret fund to Talmadge's family members and the way the cash was requested.

Minchew said the statements were part of his understanding with Talmadge that the scheme was to be covered up. But he declined the suggestion by Talmadge's attorney that they were lies. Instead, he called them "less than candid."

"It was an effort to buy time, to mislead [the Justice investigators]," said Minchew, "so I could get instructions from my principal as to what we were going to do."