Last March, flying home to Kentucky, Sen. Mitch McConnell, a conservative Republican in his second month in the Senate, read a national news magazine account of the growing violence in South Africa.

McConnell, who graduated from the University of Louisville in 1964 just as the civil rights movement reached its peak with enactment of the Voting Rights Act, says he was struck by the basic justice of the cause of South African blacks in their struggle against apartheid.

Back in Washington, the new senator asked his staff to explore the issue. Within a month a bill to impose economic sanctions against the white-minority government of South Africa was introduced by McConnell and Sen. William V. Roth (R-Del.). This legislation was among numerous sanctions bills from which were forged the measure that forced President Reagan to reverse his longstanding opposition to sanctions.

McConnell's decision to seize on South Africa for his first major foreign policy foray was among the many small signs that the Reagan administration's policy of "constructive engagement" in South Africa was being swamped in a rising tide of public and political dissatisfaction.

Long an issue of deep concern to U.S. blacks and Democratic liberals, the plight of South Africa's black majority began to draw the attention of lawmakers like McConnell and some of his equally conservative Republican colleagues in the House.

"Bipartisan" became the watchword to describe the rising tide, and by the time it crested late this summer, Reagan and the other architects of constructive engagement had been forced into a politically untenable position. Faced with certain defeat in the Senate, Reagan, in McConnell's words, "completely caved," abandoning his longstanding policy by signing an executive order imposing a milder form of sanctions than those contained in the sanctions legislation that had been headed for overwhelming approval despite a veto threat.

Democrats, who charge that the Reagan executive order is riddled with loopholes, would disagree sharply with McConnell's description of the presidential about-face. But whether Reagan's limited sanctions are seen as a genuine new policy initiative or a ploy to buy time for a failed policy, there is widespread agreement that a fundamental shift has taken place in public and congressional sentiment and in the terms of the debate over U.S. policy toward South Africa.

"The executive order transformed the issue from whether to apply sanctions to which sanctions are the most effective," said Rep. Stephen J. Solarz (D-N.Y.).

How this came about -- how South Africa, apartheid and sanctions moved to the forefront of the American political stage in the summer of 1985 -- is a case study in the rise of political forces in the age of global media coverage.

Widely scattered events in Washington and Pretoria converged over a relatively short time and were linked in U.S. living rooms by simultaneous media coverage. Prominent politicians such as Sen. Edward M. Kennedy (D-Mass.), who made a highly publicized trip to South Africa in January, played important roles, but so did protesters who were arrested outside the South African Embassy in Washington, and anonymous black youths who were killed by security forces during violent demonstrations in South Africa.

Finally, a new generation of conservative Republicans, arguing that there was no future for themselves or their party in being seen as defending the South African government, joined the liberal Democrats in demanding something more than constructive engagement.

Whether Reagan, vacationing at his California ranch in August, was aware of the forces closing in on his policy is not known, but by the time he returned to Washington in early September he had been isolated from all but the most extreme elements of his party in Congress. The executive order followed a short time later.

Nothing was particularly new about the issues, and for years liberal Democrats, such as Solarz, a former chairman of the House subcommittee on Africa, had been trying to get the public to pay attention. Still, South Africa and the proper U.S. response to apartheid remained far in the political background.

Several factors contributed to the rise in public awareness of the issues, Solarz said, beginning with the award of the Nobel Peace Prize last October to Bishop Desmond Tutu, a moderate spokesman for South Africa's black majority who suddenly found himself with a vastly enlarged audience.

A month later, a group of U.S. black leaders launched a movement that had an even larger impact on the political debate surrounding the South Africa issue. Alarmed by the deteriorating situation in that country, they formed the Free South Africa Movement and began a daily series of demonstrations and acts of civil disobedience outside the South African Embassy here. Over the next nine months, more than 3,000 antiapartheid demonstrators -- including 22 members of Congress -- were arrested in the peaceful protests, which were usually recorded by television camera crews. The demonstrations spread to other cities.

From the beginning, said Randall Robinson, executive director of TransAfrica and national coordinator of the Free South Africa Movement, the protests were part of a two-track strategy aimed at forcing a shift in U.S. policy. While the protests and daily arrests generated growing public awareness of the issues, antiapartheid leaders in Congress pushed sanctions legislation.

The Reagan administration was pursuing its constructive engagement policy as quietly as possible, Robinson said, and the key was to bring the issues to public awareness. "We never doubted from the beginning what the public's response would be," he said.

The direction of this strategy was not immediately clear to even a veteran of the antiapartheid movement such as Solarz.

"When they first embarked on this, it wasn't clear to me how violating the laws of the District of Columbia was going to bring about changes in South Africa," Solarz said. "In retrospect, they were far more effective than all my speeches on the floor of the House."

The parallels between the tactics of the Free South Africa Movement and the U.S. civil rights movement in the 1960s were obvious and did not go unnoticed by a group of young conservative House Republicans, among them Rep. Vin Weber (R-Minn.), chairman of the Conservative Opportunity Society and Solarz's ideological opposite.

"We talked a lot about the South Africa issue," Weber said. "A majority felt that if this is going to be a high-profile issue, we don't want to be in the position of defending South Africa in a knee-jerk fashion."

In December, Weber and other members of the Conservative Opportunity Society sent a letter to the South African ambassador warning that they would support a tougher U.S. line unless more progress was made toward dismantling apartheid.

The conservatives' letter, Robinson said, was "extremely important" not only in broadening support for action on the issue but also in "projecting a message to the nation that this is not a squabble between parties for political gain but people saying together that apartheid is wrong."

Weber and the other young Republican conservatives were motivated not only by strong personal opposition to apartheid but by the stakes for their party if it were to achieve majority status. They were particularly concerned about the impact on young voters if Republicans were seen as clinging to an increasingly discredited policy.

In the Senate, McConnell argued to colleagues that he owed his election to younger voters, many of them supporting a Republican for the first time, and that there was no question about where these voters stood on the apartheid issue.

"The issue is of symbolic importance to the younger generation, black and white, and has the most potential to radicalize the campuses again," McConnell said.

Weber, 33, and McConnell, 43, recalled living through the civil rights struggles of the 1960s in explaining how they read the political landscape surrounding the South Africa issue, particularly as it would be seen by younger voters.

"All of us under the age of 40 were politicized during that time we had the great civil rights struggles of the 20th century," Weber said. "There is a consensus among the under-40 voters that that struggle was justified and significant. No serious person in our generation argues about civil rights and the role of government."

Said McConnell:

"In the 1960s, when I was in college, civil rights issues were clear. After that, it became complicated with questions of quotas and other matters that split people of goodwill. When the apartheid issue came along, it made civil rights black and white again. It was not complicated."

That is precisely the calculation that Robinson and other leaders of the Free South Africa Movement made in November -- that once the issue moved to the political forefront there was no question about the public's response.

According to Ann Welch, research director of the Conference on Issues and Media, a private organization based in Alexandria that monitors national news coverage of political issues, South Africa -- events there and related political developments in the United States -- has dominated the national news agenda since mid-July. In the two weeks ending Sept. 8, which was the day before Reagan signed the executive order, South Africa accounted for 11.5 percent of national news coverage, more than double the attention paid to the second-ranking issue, U.S.-Soviet relations, Welch said.

"The American audience has to be primed to really look at another place in the world," she said. "It's the tying of issues overseas to events in this country. The fact that black leaders started the protests last November got the public's attention, and quickly a lot more people were willing to look at South Africa beyond the violence they saw on television. Even before the violence escalated, you had a lot of people saying we have to make a decision on this issue."

According to Robinson, the "priming" of the American public on South Africa has only begun. The protests and the pressure for additional change in U.S. policy will continue, he said.

But although sharply critical of Reagan's response to the events of the last 10 months, Robinson said he sees in the broad support generated by the antiapartheid movement even among conservative Republicans such as Weber and McConnell evidence of eventual success.

"I think we can measure progress made and not made over the last 20 years by the behavior of the nation on this issue," he said. "National values were improved by the civil rights movement, a beachhead was erected. I think the nation does not want to go back to what we were. This is a clear measure of how little we want to go back."