Veteran newsman Dan Rather (search) announced Tuesday that he would step down as anchor of "CBS Evening News" in March, the 24th anniversary of his taking over the job from Walter Cronkite.

The move comes just months after Rather, 73, was taken to task for going to air with a controversial "60 Minutes II" story that questioned President Bush's service in the National Guard, a piece that turned out to be based on allegedly forged documents.

Rather said he would continue to work for CBS News as a correspondent for both editions of "60 Minutes."

"I have always been and remain a 'hard news' investigative reporter at heart," he said. "I now look forward to pouring my heart into that kind of reporting full time."

Rather spoke to his viewers about his exit midway through his evening newscast Tuesday.

"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.

Rather made no mention of the National Guard story in announcing the change, saying he had agreed with CBS executives last summer that the right time to leave would be after the Nov. 2 election.

"I think he's probably ready to do it. It's a good thing to do. He's about 15 years after [Walter] Cronkite stepped down ... so he's had a good run and it's been great," CBS colleague Andy Rooney told reporters.

David Blum, author of this year's "Tick... Tick... Tick...: The Long Life & Turbulent Times of '60 Minutes,'" told FOX News that CBS had wanted Rather to step out of the anchor chair for some time.

"CBS always wanted, at least in recent years, to move Rather out, bring in a successor and shore up the ratings. They've been in the third position for a while now. Nothing really happened that wasn't anticipated," he said.

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, though the network hasn't indicated any preference for a replacement.

Network executives praised Rather following the announcement Tuesday.

"He has been an eyewitness to the most important events for more than 40 years and played a crucial role in keeping the American public informed about those events and their larger significance," CBS Chairman Leslie Moonves (search) said.

CBS thought it had an important scoop with the National Guard story this past September, 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," Rather said, "and for that I am sorry."

A report on what went wrong with the National Guard story, from a two-man independent investigative panel, is due imminently. Rather reported the story and initially defended it when it was criticized.

Rather's announcement came eight days before his NBC rival, Tom Brokaw (search), was scheduled to step down as "NBC Nightly News" anchor and be replaced by Brian Williams (search).

The triumvirate of Rather, Brokaw and ABC's Peter Jennings (search) has ruled network news for more than two decades. Rather dominated ratings after taking over for Cronkite in 1981, but he was eclipsed first by Jennings and then by Brokaw. His evening news broadcast generally runs a distant third in the ratings each week.

Rather told The Associated Press that the guard story had nothing to do with his announcement.

"Everybody will have their own thoughts about this, but ... this was a separate decision apart from that," he said in an interview.

Discussions with CBS management about when he would leave began in 1999, were shelved after the Sept. 11 terrorist attacks and then renewed last summer, Rather said. He said he and Moonves agreed his departure would be sometime early next year and Rather settled on March 9 — the 24th anniversary of when he succeeded Cronkite.

CBS News and Rather were undoubtedly weighing whether timing the announcement before or after the investigative panel's release would be better, said Ken Auletta, media columnist for The New Yorker magazine.

"I'm sure one of the things that Rather was doing here was thinking about his legacy," Auletta said. "It must be frustrating for a guy like this who has spent 24 years doing this and building up his career to be tainted by an event that he didn't have control over."

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 was also 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. 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."

ABC News said Jennings was traveling and could not immediately be reached for comment.

About his successor, Rather said, "I hope it'll be somebody from the inside. 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."

The transition is likely to raise renewed questions about the long-term viability of evening news broadcasts, which have been suffering from declining ratings for years in a world of instant Internet and cable news.

Rather has long been a target of critics who accused him of a liberal bias, and there's even a Web site devoted to that notion. The National Guard story sent those critics into overdrive. Rather's announcement Tuesday led one Republican congressman from Pennsylvania to issue a statement saying, in effect, good riddance.

"Dan Rather has been a legend in media for more than a quarter-century to many people around the world, but not to me," Rep. Bill Shuster said. "For the entirety of his career, Rather has allowed his liberal bias to shape the news rather than report it."

But while the guard incident has clearly hurt Rather in his final months on the job, CBS News President Andrew Heyward said he hoped viewers would understand the longstanding newsman's place in broadcast history.

"He's covered every story on a national basis since the Kennedy assassination, and anybody who's looked at his legacy in a fair manner is going to see the larger context," Heyward said.

Alex Jones, director of Harvard University's Shorenstein Center on Press, Politics and Public Policy, agreed.

"I think Dan Rather has been the embodiment of the indefatigable and high-powered broadcast journalist," Jones said. "I respect him highly."

The Associated Press contributed to this report.