Roger Mudd, widely considered to be the best political reporter in broadcast journalism, has been dropped as coanchor of the "NBC Nightly News" effective Sept. 5 because of lagging ratings. The network demoted him to occasional documentaries and the newly created position of "senior political correspondent."

The decision, made last week and revealed yesterday, came 15 months after the team of Mudd and Tom Brokaw took over the "Nightly News" amid considerable network fanfare and oft-stated high hopes. Brokaw anchored the newscast from New York and Mudd from Washington, much as Chet Huntley and David Brinkley had done at NBC in the 1950s and early '60s.

Brokaw will now be sole anchor of the program, which has failed to achieve competitive ratings as quickly as NBC expected. "NBC Nightly News" and "ABC World News Tonight" take turns in second place in weekly ratings, always behind "The CBS Evening News With Dan Rather."

Yesterday's announcement immediately raised speculation that Mudd will leave NBC News for ABC News--whose chief anchor, Frank Reynolds, died last week--as soon as his NBC contract allows. Mudd did not actively discourage the rumors.

Mudd, who broke the news to his coworkers at NBC's Washington bureau at noon yesterday, said during an interview just before that meeting that he felt NBC's decision was "absolutely" wrong and decried the absence of a regular Washington presence on the program.

"That's my major disappointment," Mudd said. "I think it's important for people to see the government operating, have public officials held accountable for what they're doing. It's valuable, it's necessary, it's mandatory. There's a built-in anti-Washington bias among network management. They think most Washington news is 'inside' and boring. I think it's the most important news city in the world."

Mudd, who has covered Washington for more than two decades--first for CBS News, later for NBC--said he feared network news was moving in the direction of "less and less substance, more and more Kilauea volcanoes and Diana Ross concerts," referring to two puffy stories that have recently popped up on network newscasts.

"There's a line from the show 'A Chorus Line,' " Mudd said. " 'God, I'm a dancer; a dancer dances.' Well, a reporter reports. That's how I feel. I want to work."

NBC's abrupt and unexpected move was, for Mudd, reminiscent of the incident that caused him to leave CBS in 1980, when Dan Rather, instead of Mudd, was chosen to succeed Walter Cronkite as anchor of the top-rated "CBS Evening News." Mudd said later it wasn't the decision itself so much as the way he was belatedly informed of it that insulted him.

Yesterday, Mudd, who reportedly earns more than $500,000 a year, said he did not consider being kicked off the "Nightly News" that kind of insult. "I'm not humiliated by it," he said. "I did nothing shameful or unconscionable. I did what I was called on to do." Mudd said the only reason for the change given him by NBC News president Reuven Frank, when Frank flew to Washington with the news Thursday, was that it was a "subjective" decision.

"They became impatient about the ratings," Mudd said. "They were feeling pressure from the corporate side and the affiliates. Things were not moving the way they hoped. The decision right at the bottom line, you'd have to say, would be based on the ratings."

Asked if that was the reason for the change, Frank said yesterday from his office in New York, "I will not answer that." Asked about industry speculation that Brokaw had been pressuring NBC News for months to make him sole anchor on the program, Frank said, "If so, he hasn't done it with me." Frank also denied that pressure from affiliates or from corporate brass forced him to make the change, saying, "I accept full responsibility," while conceding the decision was made jointly with NBC chairman Grant Tinker and NBC president Robert Mulholland. "The decision came out of a few television professionals looking at a television program and saying, 'That's a good show, why doesn't it work?' " Frank said. "The reason it didn't work was the two-man anchor. The rest of the decision which anchor would be dropped was much harder to make."

In his new role at NBC, Mudd, 55, will be host and principal reporter for eight "NBC White Paper" documentaries to air in the coming season (an increase from the four aired last season), and will be "an important reporter," Frank said, in NBC's coverage of the election year ahead, though he will not have coanchor status of that coverage with Brokaw, who is 12 years his junior.

Although Mudd is known to dislike the razzle-dazzle style of ABC News president Roone Arledge, he did not rule out yesterday the possibility that he might join forces with Arledge once his contract permits it. ABC News is currently strapped for high-profile on-air personalities. Mudd said his agent, Ralph Mann, was tentatively approached by ABC News vice president David Burke, Arledge's right-hand man, after recent rumors that Mudd would be moved from "Nightly News" to the NBC magazine program "Monitor."

Mudd said a "window" in his contract permits him to investigate other employment possibilities six months after his last "Nightly News" program. He said he would be "receptive to all offers--offers are very flattering" but also said he would not consider returning to CBS News. That only leaves one network, ABC.

Mann said Mudd's contract with NBC is "a very, very long-term contract that has a lot, lot longer to go" (it expires in 1991, Mudd said) but that "there is a window. It will all be discussed in good faith."

ABC News vice president David Burke said yesterday that terms of Mudd's contract and whatever "windows" it has are not known at ABC News, but as for the possibility of Mudd receiving overtures from the network, "It certainly isn't something to rule out." Burke said ABC News spoke with Mudd about coming there when Mudd left CBS in 1980 and before he signed with NBC News. Talks broke off when it became apparent Mudd was gravitating toward NBC News and its president at the time, William Small, who had been CBS News Washington bureau chief and Mudd's boss.

Broadcast journalists at NBC and other networks expressed either shock or incredulity yesterday when they learned of the NBC decision. One top network newsman said, "It's one of the dumber moves made in the Byzantine world of network news. NBC brought Roger Mudd in at top dollar, kept his visibility very high, and at the very moment when Roger's strengths could pay off for NBC, they dump him." The source said the political year for the networks begins just after Labor Day, which will be when Mudd steps down as coanchor.

NBC corporate vice president Meryl S. Rukeyser Jr. said Mudd was not being treated shabbily by NBC. "He's not exactly being thrown out on the street," Rukeyser said. "This has been handled in a sensitive way, out of a sincere desire to keep Roger working, and working as happily as he can here." In a statement, Frank called Mudd "one of the best political reporters in the country and a consummate journalist."

In an accompanying statement, Brokaw said, "Roger is a friend and a journalist for whom I have the highest regard." Mudd said of Brokaw, "Our relationship has always been first class. It really has."

Mudd is thought of so highly at NBC, one insider said, that he was allowed to compose much of the press release announcing the change. Rukeyser would not confirm that report.

Although he looked wounded and shaken yesterday when discussing the change, Mudd expressed no bitterness toward NBC News, only toward corporate management of NBC. He said he preferred working at NBC News to his days at CBS. "It's a sweeter, kinder place," Mudd said. "More relaxed. Not as brittle. People seem to care more about each other." He also said that getting back to "first-hand reporting," as opposed to anchoring, with a "good, vigorous first-rate documentary unit" might be satisfying to him.

"I've believed in being serious about my work and not putting any shellac on stuff," Mudd said. "I still believe that. And I think there are enough people in this business who believe in the principles of journalism that I do, so that they're going to come back some day--that there is a place in American television for good, steady, serious journalism."

Mudd said that technically NBC had violated a clause in his contract that prevented the network from moving him out of the coanchor chair until the broadcast had been on the air for two years; NBC acted nine months prematurely. But he said the only way to fight the decision would be to take NBC to court, a long and excruciating process. He also said he had no intention of staying home and refusing to work, as he did for a few months before his CBS contract officially expired in 1980.

In reflecting on Mudd's fate, Burke said yesterday of network news, "It's a strange business." And Mudd, contemplating the swiftness with which another ax had fallen on him yesterday, smiled and said, "It's a speedy business, isn't it?"