PHOENIX, MARCH 1 -- Attorneys for Arizona Gov. Evan Mecham (R), seeking to discredit a key witness against the first U.S. governor impeached in 59 years, threw a political grenade into his state Senate trial today with a charge of sexual misconduct against the state police chief.

In a deposition distributed to senators Monday by Mecham's lawyers, a former employee of public safety director Ralph Milstead alleged that he had an affair with her and then threatened to "take care" of her if she told anyone.

Attorney Fred Craft, in his opening statement on Mecham's behalf, referred to the deposition and said it was important to "question the integrity" of witnesses who "have a stake" in seeing the governor ousted. Milstead is expected to testify on the most dramatic and personal charge against Mecham: that he ordered Milstead not to tell the state attorney general about a threat made by a Mecham appointee against a grand jury witness.

Milstead called the deposition by a former Department of Public Safety investigator "a ridiculous bunch of lies." Sen. Jaime P. Gutierrez (D-Tucson) described the defense's use of it as "hard ball."

Gutierrez said Milstead's character is an important issue but said he does not know whether the deposition will make a difference. A senator who asked not to be named said he thought it would encourage Mecham's Senate supporters but offend senators who have taken the middle ground.

The Senate trial, expected to last two months, is one of three major challenges to the governor. He and his brother, Willard, are scheduled to go on trial in Maricopa County Superior Court March 22 on charges of fraud and perjury in connection with an unreported $350,000 loan to his 1986 campaign.

And more than 375,000 voters signed a recall petition forcing him to run against challengers in a recall election May 17.

The state Supreme Court today rejected Mecham's request to delay the Senate trial until after his criminal trial. He is expected to renew the request in federal court.

Mecham was removed from office temporarily after his Feb. 5 impeachment by the state House. The Senate began hearing testimony today on the House charge of obstruction of justice and will later take up allegations that he failed to report a campaign loan and an improperly loaned $80,000 in state funds to his family's automobile dealership.

William P. French, a former Phoenix judge serving as chief prosecutor in the Senate, summarized in his opening statement the November 1987 events that led to the obstruction charge:

French said Lee Watkins, an assistant director of the department of administration, told Peggy Griffith, director of the governor's office of women, that a former Mecham aide was hurting the governor by testifying to a grand jury investigating the unreported campaign loan. Griffith told investigators that Watkins had said the aide "would be going on a long boat ride and may never come back.

"It is very likely not to happen right away. It could happen next spring or fall. It didn't matter if she moved to Wisconsin and changed her name, she could still be found," Watkins said, according to Griffith's account.

Griffith reported the alleged threat to a DPS officer, and Mecham later asked that the matter be referred to Watkins' boss, department of administration director Max Hawkins.

But Milstead, according to French, pressed to refer the matter to the attorney general. In a Nov. 15 telephone conversation, French said, Mecham told Milstead, "The attorney general is out to hang me, and I am not going to help him in any way."