For one bright shining moment way back about 12 days ago, Ronald Reagan was looking like an easy winner in the 1980 presidential election. Then Reagan began his campaign; the shine and the moment quickly crumbled.

Well, so it looked in the nation's mass media, anyhow. In just a few days the Reagan camp sent out a series of harsh and confusing messages that produced a cold bath of negative publicity, and the pollsters announced (coincidentally) that President Carter was pulling even with Republican challenger. Suddenly, the prognoses of a Carter defeat that were commonplace at the end of the Democratic convention disappeared.

Reagan's first 10 days on the campaign trail provide a textbook lesson in political communications in the mass media age. To follow those 10 days as they were recorded in newspapers, magazines and on television is an intriguing exercise. It reveals both the inadequacies of the news coverage and also the ways in which Americans get their messages from a political campaign.

The consequences of those rocky 10 days could be devastating to Reagan, or they could disappear entirely before November. All that can be said with confidence now is that Reagan squandered the initiative and revived the one issue that can probably hurt him most -- his own plausibility as a president -- during the opening days of this campaign season.

The first event of this period was a news conference that Reagan and his running mate, George Bush, held in Los Angeles on Aug. 16. This was Reagan's first public appearance since the end of the Democratic convention, but its principal purpose was to give Bush a formal sendoff on his trip to Japan and China, a journey the Reagan camp designed to demonstrate the Republican ticket's diplomatic experience and ability.

Like many of Reagan's appearances during these 10 days, that news conference offered reporters covering it at least three different "leads" or opening sentences for their reports. All three were used by different news organizations.

The first was probably the lead the Reagan organization wanted: "Ronald Reagan and George Bush . . . took steps today to assure China that a Reagan administration would make no change in Sino-American relations." That was The Washington Post's version, written by a member of The Post's staff in Los Angeles.

The New York Times (in a dispatch printed by many other newspapers) led with Reagan's assurance that despite their convention rhetoric, the Democrats would be unable "to portray me as a combination of Ebenezer Scrooge and the Mad Bomber" during the campaign.

But the real of the story, as it turned out, was the one the Los Angeles Times and CBS News used: "Ronald Reagan repeated his intention to reestablish official government relations with Taiwan Saturday . . ." as the L.A. Times put it.

It took several days for all the news media to catch on to the significance of that fact, but eventually it became a dominant element in the bad publicity for Reagan that was to follow.

Meanwhile, the candidate set out on a week of campaigning that would include major speeches to the American Legion and Veterans of Foreign Wars, and a tour of a shipyard in Philadelphia. During these appearances, Reagan's handlers planned, the candidate would set out clearly his position on the great national security issues of the day.

At the first stop -- the VFW convention in Chicago -- this plan went awry. Reagan's speech there was formally entitled "PEACE: Restoring the Margin of Safety," and it was meant to send the message that Reagan as president would maintain a genuine peace by rebuilding America's armed forces. pBut it didn't come out that way in the mass media.

"Reagan Defends Viet War" blared the banner headline in the next day's Chicago Tribune. The evening news on both CBS and NBC began with the same bit of news, and ABC featured it prominently. So did the Associated Press and United Press International, the two wire services upon whom most daily papers depend for national political news.

Reagan had personally added a passage to his VFW speech describing the Vietnam war as a "noble cause" that the United States had been "afraid to . . . win." It was red flag to the news media, one that distracted most of the reporters covering the speech from the principal message Reagan wanted to convey.

Even that message got confused, largely because reporters couldn't sort out the calls for peace from the calls for more arms. This problem persisted for the next two days as Reagan continued to speak out on defense matters. The hawkish tone set by the first day's ringing defense of the Vietnam war and "exporting Americanism" seemed to color the coverage Reagan got all week.

Meanwhile, the import of that Aug. 16 news conference began to register. On Tuesday night, Aug. 19, all three networks carried reports suggesting that Bush -- then still in Tokyo -- faced serious problems in Peking because of Reagan's pledge to restore "official government relations" with Taiwan.

The Chinese delivered a blistering attack on Reagan -- "They're hopping mad," as NBC's Nightly News put it -- and Bush was on the defensive. CBS quoted Reagan's principal foreign policy aide as "backing off" from Reagan's latest statement, and the network's correspondent said the Republicans weren't conveying the polished image the trip was meant to project.

Also Tuesday, the new Gallup Poll was released for publication in Wednesday's papers. In The Washington Post's Wednesday editions the news that Carter has pulled even with Reagan by Gallup's reckoning appeared beside an article by Lou Cannon, a reporter who has been covering Reagan for more than 15 years and is following his campaign this year for The Post:

"Ronald Reagan campaigned today in gloomy weather," Cannon's dispatch began, "which matched some of the omens for his own candidacy." Cannon went on to say that Reagan aides were acknowledging that the former governor had failed to convey the clear positions on national security issues that they had intended.

A dispatch like Cannon's carries substantial but incalculable weight inside the news media. A senior political reporter, known as an expert on Reagan, writing in one of the two newspapers that virtually everyone in the media reads, can color reporters' and editors' views of a situation almost subliminally. Cannon's story probably contributed substantially to the general perception that Reagan was having a bad week.

There were other, contrary messages the same day. The Wall Street Journal, for example, editorialized that Reagan did just the right thing by declaring Vietnam a noble cause. "It's about time," the Journal said.

The China flap continued to intensify. ABC's "World News Tonight" on Thursday reported Bush's continuing problems in China, then showed Reagan in Los Angeles reiterating his view (which Bush had been trying to "explain away," according to ABC) that there should be an official U.S. office on Taiwan.

Over the weekend all three networks were routinely reporting the Bush trip and cacophomous Reagan comments in the United States as a flub and an embarrassment for the Republican ticket. This had become conventional wisdom by Sunday night -- there was no other version of the event on the networks.

Curiously, the Chinese government gave the American media a way to make this point. Its repeated official denunciations of Reagan and Bush were carried withou comment or refutation, including the final Chinese judgement that Bush's visit was a failure By inference, the networks seemed to agree.

Over the weekend, the columnists and editorialist began to comment on Reagan's week. The Monday Washington Post carried an editorial saying Reagan had not really made a grave goof over China, but the overriding message of other comments was that the Republican had shot himself in the foot.

This set the scene for the final act of this unhappy little drama for the Reagan campaign -- Monday's news conference in Los Angeles. By its very nature this was a negative event for Reagan -- an attempt to explain what had happened, to put a favorable light on unfavorable events.

As it turned out, one network -- NBC -- gave Reagan something of a break, reporting his news conference without pointing out the contradictions between Monday's statements and earlier public pronouncements.

But the two other networks, ABC and CBS, gave their viewers a much sharper account. ABC juxtaposed film clips to show Bush flatly contradicting Reagan on one key point, and both networks showed Reagan acknowledging that he "misstated."

"The controversy is not likely to blow away," concluded the ABC correspondent, Barry Serafin, "controversy which has clouded what was meant to have been a showcase for George Bush's credentials and the foreign policy prowess of the Republican ticket."

Contemplating these events this week, Reagan's camp might take heart from one media oversight. The major news organizations -- perhaps preoccupied by the China flap -- made little or nothing of Reagan's potentially controversial statement, delivered Friday, that he had "a great many questions about" the theory of evolution. "I think that recent discoveries down through the years pointed [out] great flaws in it," Reagan added, referring to the theory first propounded by Charles Darwin.