Sheila Tate, Nancy Reagan's press secretary, who helped turn the first lady's image into that of a national spokeswoman against drug abuse, resigned yesterday to join Burson-Marsteller, the nation's largest public relations firm, as a senior vice president in its Washington office. No replacement has been named.

"It was the hardest career decision I have ever made. I have loved working for Mrs. Reagan, but four years was enough for me, this was right for me," said Tate, 42, who told Mrs. Reagan a week ago of her decision to leave.

In a statement released by the White House, Mrs. Reagan was quoted as accepting Tate's resignation "with deep regret.

"I will always value the hard work and dedication Sheila brought to the position of press secretary. I will truly miss her," Mrs. Reagan said.

Tate said she told Mrs. Reagan she will stay on until a replacement is found, probably through early February. "We'll have some overlap," Tate said.

Other White House sources said Tate is "pulling together the list" and that there are a number of candidates under consideration, "some known in the news community here and one or two who are not."

Tate said she decided to leave her $53,900-a-year White House job for one in public relations at an undisclosed salary ("That's the joy of the private sector. I don't have to tell you," she said) shortly after the first of the year.

"I would have preferred to announce it after the inaugural but I started hearing reports about it. Press secretaries have to notify everybody at once so I suggested today that since there were stories circulating, it was time to put it out, and Mrs. Reagan agreed," Tate said.

Jon Jessar, senior vice president and general manager of the firm's Washington office, said he began offering Tate a job four years ago.

"I simply always told her that when she was ready to leave I would love to talk to her," Jessar said. "I don't know if it Nancy Reagan's image needed turning around but I think Sheila did an outstanding job of doing what needed to be done."

Jessar also confirmed that his firm "would love to have" White House deputy chief of staff Michael K. Deaver. "There'll be a discussion, unquestionably," said Jessar. Although there has been wide speculation that Deaver would join Burson-Marsteller, other sources say Deaver is considering other options, including opening a firm of his own.

James Rosebush, Nancy Reagan's staff director, whose relations with Tate have not always been smooth, said he was "very surprised when she came to tell me a couple of days ago."

Saying he was "sorry to lose her," Rosebush said, "we all sort of grew up together in the sense of what was going on here in drug abuse. We all did it together."

He said, "I think we had a good working relationship, which grew better as time went on. She was a straight shooter."

Washington consultant Nancy Reynolds hired Tate as Nancy Reagan's press secretary four years ago this week after a brief stint in the job by a California reporter proved unsuccessful. Yesterday, Reynolds said "the wonderful publicity surrounding Nancy Reagan's drug involvement is a credit to Sheila. She did a good job and it reflected well on Nancy. Her reliability with the press also reflected on Nancy."

White House spokesman Larry Speakes, who knew Tate when both worked at the public relations firm of Hill and Knowlton, said she was "a top-notch public relations professional who has been a tremendous asset to Mrs. Reagan and the White House."

Speakes said one of the things he was proudest of was that the first lady's East Wing side of the White House and the president's West Wing press offices "have gotten along without a ripple for four years."

No such harmony marked some of their predecessors. Richard Nixon's press secretary Ronald Ziegler rarely communicated with Pat Nixon's press secrtaries. Gerald Ford's press secretary Ron Nessen and Betty Ford's spokeswoman Sheila Weidenfeld were frequently at odds.

White House social secretary Gahl Hodges said she was not surprised by Tate's decision to leave.

"Four years is long enough in any job. Even though it's a great honor, you're no longer learning anything new, it's no longer a challenge," Hodges said. "There's a burnout factor, too. When I got here things were clicking along."