Dan Rather's (search) remaining months spent on-air as leader of CBS News (search) may be spent trying to ensure that his 42-year career there isn't defined by the network's report on President Bush's National Guard service.

Rather, 73, announced Tuesday he would step down in March as the anchor of the "CBS Evening News" (search) after nearly a quarter-century in the job, and become a correspondent for both editions of "60 Minutes."

He said his decision had nothing to do with the independent investigation into the September National Guard story and took pains to separate the events by saying he and CBS management had periodically discussed his stepping down since 1999.

"I'd like to put some space between this decision and whatever the panel comes through with," Rather said in an interview.

His agent, Richard Liebner, said it would be a shame if the Guard story becomes Rather's defining moment.

"I would hope that people appreciate and understand the longevity of his time in the chair and the breadth and scope of his career and all the major stories he's broken," Liebner said.

Rather replaced broadcast legend Walter Cronkite in 1981 and lasted even longer than his predecessor's 19 years. Rather, Tom Brokaw of NBC and Peter Jennings of ABC competed at the top ranks of network news for more than two decades as the world — and world of news — changed around them. Brokaw leaves NBC's "Nightly News" next week.

His career included bumps such as walking off a broadcast, an eyebrow-raising mugging and attracting ridicule by briefly signing off his newscast with the word "courage." But a "60 Minutes Wednesday" story about Bush's Guard service turned out to be based on allegedly forged documents and forced Rather to fight for his professional life.

John Roberts and Scott Pelley are frequently mentioned as in-house candidates to succeed him, but CBS News — a distant third in evening news ratings behind NBC and ABC — also will look elsewhere.

"I hope it'll be somebody from the inside," Rather said. "But whoever it is will have my complete, unadulterated support and encouragement. Probably the best way I can help is to stay out of the way."

Rather, whose Texas roots were evident in his folksy aphorisms, joined CBS News in 1962 and covered President Kennedy's assassination in Dallas a year later. He became a White House reporter in the Nixon administration and his combative style was captured in a memorable moment when Nixon, at a news conference, grumbled to him: "Are you running for something?"

"No, sir, Mr. President," Rather shot back. "Are you?"

Together with Jennings and Brokaw, Rather's continuous coverage in the wake of Sept. 11 drew praise for helping a nation come to grips with an unimaginable tragedy. He scored several scoops, including anchoring the CBS report that offered the first pictures from the Abu Ghraib prison scandal earlier this year.

But his career also was dogged by incidents that attested to a tightly wound persona. In 1987, he walked off the evening news set in anger after CBS delayed the broadcast for a tennis match, leaving the network with dead air for six minutes. Four months later, then-Vice President George H.W. Bush, angered by a line of questioning from Rather, asked if he'd like to have his career judged by the walk-off.

Rather also said he was once accosted on a New York street by a strange man who beat him, asking, "Kenneth, what is the frequency." It became an odd cultural touchstone; the rock band R.E.M (news - web sites). wrote a song about it.

"He's apt to be haunted by the bizarre things that happened to him, the mugging and everything," said Bob Lee, president of WDBJ-TV in Roanoke, Virginia, and head of CBS's affiliate board. "But he has always been a vigorous competitor and a guy who cared an awful lot about the evening news broadcast. I wish he were going out on top."

Brokaw said that he was "pleased for Dan that he's come to a conclusion about his own life, as I have in my case."

"Dan and I have known each other competitively and personally for a long, long time," Brokaw said. "Occasionally on the competitive side, it would be tiny bumps in the road, but when you think of all that we've been through, we have a pretty strong relationship. So I wish him well."

Rather dominated in the ratings after taking over from Cronkite, inheriting CBS's mantle as the most dominant broadcast news organization. But financial cuts chipped away at the institution, and Rather slipped behind first Jennings, than Brokaw in the ratings.

The Nixon and Bush incidents contributed to a long-standing suspicion of Rather in conservative political circles. There's even a Web site devoted to alleged incidents of bias, and his critics were driven into a frenzy by the National Guard story.

CBS thought it had an important exclusive with the National Guard story, reporting that President Bush had received preferential treatment to get into the guard and stay in the United States during the Vietnam War, and had failed to satisfy the requirements of his service.

But critics immediately questioned the story, saying a document purportedly written by Bush's late squadron leader appeared to be a fake. Rather apologized before CBS appointed the investigative panel.

"We made a mistake in judgment," he said at the time, "and for that I am sorry."

Former AP Chief Executive Officer Louis Boccardi and former Attorney General Dick Thornburgh, who are looking into CBS's guard story, would not comment Tuesday about the status of their report.

Rather told viewers about his exit midway through his newscast Tuesday. He's leaving on March 9, the 24th anniversary of the day he took over from Cronkite.

"It has been, and remains, an honor to be welcomed into your homes in the evening and I thank you for the trust you have given me," he said.