It is time for a change -- a good slogan in any political season, and a good description of the behavior of the mass media during the last fortnight of this political season.

Two weeks ago, Ronald Reagan was enjoying his best moment in the fall campaign, on two counts. First, the media coverage, particularly on television, was friendly and upbeat, while President Carter was getting the treatment for his "mean" campaigning. Second, a large part of the media seemed to be forming a consensus over the weekend of Oct. 4-5 that Reagan was moving into a real lead in the horse race.

It was a time for good feeling, and the Reagan camp felt good. "Optimism" was the one-word headline on a report about the Reagan command that appeared in this newspaper on Oct. 4.

Now the moment has passed, for two reasons. First, the horse race no longer appears to favor Reagan. Second, the media coverage of Reagan has turned negative. The 190 yo-yo has reversed directions once again.

Both changes deserve scrutiny. Both are instructive examples of the dynamics of a presidential campaign in the mass media era.

If there is a first law governing media behavior in an election campaign, this may be it: Never fall behind a trend, never miss a new situation. Following this law, rreporters, producers and editors trip repeatedly into the snapshot trap. They take new snapshots constantly, then periodically misinterpret one of them as a genuine portrait.

The snapshot 10 days ago showed Reagan leading in enough states to win the election, at least according to the surveys done by NBS News, Newsweek, The Washington Star, The New York Times. (All of them appeared during a few days, on or near the weekend before last.) The snapshot showed Carter struggling with a spreading perceptio that he was campaigning nastily, meanly, unpresidentially.

But this was just a snapshot, not an accurate portrait of the complex American organism at the height of a presidential campaign. And virtually the moment it was developed and pasted into the media album, it started to turn brown. Within a couple of days it was thoroughly out of date; now, 10 days later, it has been replaced with a new snapshot.

During exactly this same period, the tone of the media coverage of Reagan -- particularly on television -- has changed from passive and friendly to much more skeptical. Arguably, the change in perception of Reagan's chances affected the perception of Reagan the candidate, but in fact, there were three separate developments beyond the perception of the horserace that came into play.

First, the mass media was coming to the end of a cycle in its campaign reporting. After more than a week of friendly, upbeat Reagan coverage, reporters, producers and editors were due for one of the periodic moments of re-evaluation that is typical of their trade. Shouldn't we be taking a closer look at Reagan now? That question was asked at The Washington Post, and at other media offices, too, particularly CBS News, about which more in a moment.

Second, the White House -- in the persons of press secretary Jody Powell, pollster Pat Caddell and others -- had begun a concerted effort to put pressure on the media, pressure to end what they called Reagan's "free ride" on television and in the papers. These Carter aides approached reporters and editors personally, complaining that Reagan's record deserved much closer scrutiny than it was getting.

Third, and crucially important, Reagan himself made an enormous contribution. On Oct. 9, he caught himself in a rhetorical thicket over the sources of air pollution, declaring that Mount St. Helens and the nation's stands of timber were important sources of pollutants. Reagan looked silly on the network news shows that night, and he revived doubts about his intellectual capacities at a thoroughly (for his campaign) inopportune moment.

The first sign that the media yo-yo was beginning a new ride down for Reagan came on Tuesday evening, Oct. 7, by coincidence a disastrous day for the Carter campaign. That was the day after Carter asserted that electing Reagan would divide North from South, Christian from Jew and "rural from urban," the piece of excessive rhetoric that finally enabled Carter's handlers to convince the president he was going too far.

On the network news that night, viewers saw Carter left fly with this zinger, then saw Reagan at his Hollywood best responding more in sadness than in anger. "He's reaching a point of hysteria that's hard to understand," Reagan said in a gentle, philosophic voice.

On NBC, correspondent Chris Wallace said it was the sort of episode the Reagan camp loved, because it showed "Mr. Carter as mean and unpresidential, Reagan as caring and mature." On ABC, Sam Donaldson reported: "Carter campaign officials are deeply worried tonight that the president's reelection is slipping away . . .

CBS gave its 25 million viewers a negative account of Carter's day, too, then added one of the strongest pieces of television journalism that has appeared on the networks all fall.

It was a report by Bill Plante, one of television's best reporters, who has covered Reagan all year. Since this campaign began, Plante reported, "Ronald Reagan has been shifting from the right to the center of the political spectrum."

Plante cited five examples, using film clips with most of them showing Reagan saying something during last winter's primaries or earlier that he is no longer saying today. For example, Reagan used to talk about getting "the federal government out of the classroom," and of abolishing the departments of Education and Energy.

"No more is heard about that," Plante said, while on the screen a big white X was drawn electronically through the face of the Reagan who had just spoken.

Plante went on, citing Reagan's shifting positions on abolishing the inheritance tax, subjecting big unions to antitrust laws, aiding Chrysler and New York City. The big white Xs kept on obliterating pictures of the old Reagan. Then Plante closed the report like this:

"Which is the real Ronald Reagan? Does he plan to deliver on his conservative promises? Or is he really a closet moderate? His aides say that it's simply that he understands the politics of getting elected. In any case it presents President Carter with the problem of convincing voters that he's talking about the same Ronald Reagan who looks and sounds so much more moderate today."

This is tough stuff for television. CBS switchboards lit up with complaints from viewers who thought the Xs obliterating Reagan's face were unfair. (The next night, Walter Cronkite apologized to viewers for this "graphic," but not for the report itself.)

Forty-eight hours later all three networks were showing "negative" Reagan material, most of it based on the self-induced confusion over air pollution. In quick snippets of videotape, viewers saw Reagan contradict himself twice, first saying he hadn't said that air pollution was substantially under control (after viewers say him say it), then saying he thought air pollution was indeed substantially under control.

Carter, meanwhile, had launched a new phase in his campaign after promising (to Barbara Walters in an exclusive interview on ABC, shown Wednesday night) not to be nasty any more. The networks began showing more of Carter's specific attacks on Reagan's positions and past statements, in effect helping the Carter campaign pursue its principal tactic -- raising doubts about Reagan.

By this week the whole tone of campaign coverage on the networks had changed. The CBS Evening News on Tuesday, for example, reported extensively on Carter's accusations against the Reagan economic program. Anchorman Dan Rather then noted that a Republican "truth squad" held a press conference to respond to Carter, "but the participants . . . offered little to rebut what Mr. Carter said and not much more about specific costs of Reagan's programs."