Washington — Dan Rather, America's No. 1 newscaster, launched another broadcast of the CBS Evening News with the big story of the day: ''President Reagan today told the United Nations General Assembly, 'We are ready for constructive negotiations with the Soviet Union,' adding, 'there is no sane alternative to negotiating.' ''
The next three minutes - the most crucial part of program - were devoted to Mr. Reagan's speech. The President spoke calmly of ''arms control,'' ''negotiations,'' and ''progress'' in Soviet relations. There were only a couple of negative overtones in the CBS report, from what Mr. Rather called ''political opponents.''
It all added up to a great television day for candidate Reagan. In a single appearance, highlighted on all the networks, the President managed to blunt Walter Mondale's sharp criticism on the sensitive war-and-peace issue.
Rather's handling of the President's UN speech shows how difficult it is for the news media to strike a perfect balance between candidates for the presidency. While Reagan was given lots of air time to make the key points of his address, Mr. Mondale was granted only about 14 seconds to make his response on the CBS broadcast that Monday evening.
Overall, however, campaign coverage by the three major TV networks - CBS, NBC , and ABC - has been ''almost free of ideological bias,'' according to a new study conducted at George Washington University.
Maura Clancey, assistant director of the Media Analysis Project, which is probing TV news, says that in the current campaign the networks can be given a good grade - perhaps an A-minus - on their reporting of the Reagan-Mondale race. The media project is directed by Michael J. Robinson, a professor at George Washington.
Miss Clancey says: ''We have now analyzed 161 campaign pieces appearing on network evening news during the first two weeks of the general campaign. In only one area was there any evidence whatever that the networks cover the politicians in a partisan or liberal way.''
The media project uses five tests to measure bias. The tests include: how much time a candidate gets; the tone of the coverage; the balance of quotations; obvious bias in conclusions drawn by the reporter; and how certain adjectives, such as ''far right'' and ''leftist,'' are used.
The study so far has found:
Nearly equal time. The Democratic ticket was the main focus of 70 stories, the Republicans, 69. Democrats got 5,200 news-seconds. GOP, 5,500 news-seconds.
Tone somewhat anti-Democratic. One possible reason was that the party's vice-presidential candidate, Geraldine Ferraro, came in for personal criticism from such groups as Roman Catholic bishops and others opposed to abortion.
Fair quotations. Only 4 stories of 161 studied had quotations ''out of balance.''
Few biased conclusions. Of the nine that appeared weighted to one side, six took a ''liberal'' position, three a ''conservative'' position.
Some unfairness in labels. There were 16 instances where candidates were called ''right,'' ''right wing,'' and other such emotion-laden terms. ''Leftist, '' and ''left wing,'' were never used. ''Liberal'' was used only twice.
''The networks should get credit,'' says Miss Clancey. ''Only in the labels, such as right wing, are they exhibiting bias. ... But that's not a great sin,'' she adds.
All three networks were about equal in fairness, the study found. The only unusual lapse showed up on CBS, which had six stories on Ms. Ferraro, but none on Vice-President George Bush. ''He was almost a nonentity for CBS,'' Miss Clancey notes, but she adds that this probably didn't hurt the Republicans. Some of the Ferraro stories were negative.
Interest in media coverage of politics has grown in recent years. Studies by S. Robert Lichter and Stanley Rothman have found that much of the media is dominated by people who have a secular outlook, who usually vote Democratic, and describe themselves as ''left of center'' politically.