President Reagan has an opportunity to make his own day -- a grand day for the Grand Old Party -- a great day for America and an especially great day for black Americans.

Great leadership often is not born of ideology but of action during crises, and the hot crisis now is in South Africa. Decisive, tough and substantive anti-apartheid leadership could strike a blow for freedom in a way that most of Reagan's rhetoric and action have yet to do.

It could begin to build for him the kind of mark- on-mankind presidential legacy that aides say he covets. It might begin to counter the suspicions of many black Americans that this president, bluntly put, is a racist. It could also help the Republican Party make long-sought inroads into the black vote, which many feel is essential if the GOP does become the majority party.

What would Reagan have to gain? He already has been reelected with the largest electoral landslide in American history in a campaign where he proved he didn't need black votes. He probably won't run for any other office. So why bother?

Why not? He has nothing to lose. South Africa could be for Reagan what China was for Nixon and the Civil Rights Act of 1964 was for Lyndon B. Johnson.

The book Bishop Desmond Tutu reveres is the Holy Bible not Das Kapital. Who could accuse Reagan of being soft on communism by moving against apartheid in South Africa? Who would dare suggest that Ronald Reagan, who said he would have voted against the Civil Rights Act and who nominated William Bradford Reynolds for the No. 3 job in the Justice Department, of caving in to the civil rights lobby? A Reagan manifesto on South Africa may not equal Lincoln's Emancipation Proclamation, Roosevelt's New Deal or Johnson's Great Society and Voting Rights Act -- all measures that left indelible impressions on the lives and voting habits of black Americans -- but South Africa is a central preoccupation, and the widespread opposition to apartheid makes it a pretty safe political target.

The alternative to such a mid-course correction of U.S. policy in South Africa would be to remain locked in on "constructive engagement," the approach that has earned the United States the reputation as apartheid's strongest ally.

That is a risky course for these times, however.

A generation of young blacks is emerging with only faint memories of the last great things the Democratic Party did in the fight against segregation and deprivation of blacks. More recently, this generation has memories of what the Democratic Party did not do for them in 1984, when their votes were taken for granted. Its most vivid perceptions today are of what President Reagan -- a Republican -- is doing in South Africa, and these perceptions could last a long time.

The arguments that Reagan would face are mostly tangential ones. Even British Prime Minister Margaret Thatcher can do no better than to ponder whose ox would really be gored by economic sanctions. In fact, that has been Reagan's best argument to date.

By getting tough on apartheid, Reagan might be his own leading beneficiary. On the question of principles there is now an inconsistency in Reagan's approach to, on the one hand, South African blacks and, on the other, Jews, Cubans, Afghans, Laotians and Miskito Indians in Nicaragua and all the other "victims of totalitarianism" he cited in his Bitburg speech. South Africa's blacks were conspicuously absent from the list.

He has labeled anti-Sandinistas quarreling with the Marxist government of Nicaragua as "freedom fighters" and "the moral equivalent of the Founding Fathers." By contrast, black South Africans shot by police in a March demonstration became the moral equivalent of dupes. They may not have provoked the police, Reagan said, but it was "significant" that some police were black and that there were those in South Africa simply opposed to a "peaceful solution."

On the question of race, however, there is a disturbing consistency to President Reagan. Beyond the budget cuts and his views on civil rights, Reagan has displayed a penchant for turning inane photo opportunity sessions into chances to brandish insensitivity to blacks as he has to perhaps no other ethnic group in this country. This is remarkably troublesome during a presidency that so highly values symbolism because in these instances all the symbols can be seen as anti-black.

In his 1980 debate with Jimmy Carter, for instance, Reagan portrayed the country of his youth as one oblivious to racial strife -- an America at a time when lynchings were common. He launched his campaign that year in Philadelphia, Miss., where three civil rights workers had been brutally murdered. His offhand suggestion at a press conference that Martin Luther King may have been a communist brought grimaces from top black aides on the White House staff and prompted a presidential apology to King's widow. So when at a later press conference he was asked about the March shooting in South Africa and allowed that there could have been some justifiable reason for the police to open fire, it was easy for the doubters of his own sincerity to say, "There you go again" and get a loud chorus of "Amen."

Some in Reagan's party are betting its future in part on bringing in more blacks, and the critical issue on which that could turn is inclusiveness. The Emancipation Proclamation, the New Deal, the Civil Rights and Voting Rights acts all brought blacks into America.

For black Americans -- and most blacks are very, very American -- the real insult of "constructive engagement" is that their government is on the wrong side. In all he other turning points, the government was on the black side. On the plantations of the old Confederacy, on the front lines in Little Rock and Birmingham and in the poverty-riddled ghettos of the early 1960s, the federal government came time and time again to invite blacks to share in the American dream. For many black Americans, the turmoil in South Africa gives the United States its most important opportunity to stand up for rights in the old country. And instead, President Reagan and America are on the wrong side.

There is no simple solution to the crisis in South Africa. No one can just send in the Marines, blockade the ports, yank out all the investments, cut ties and assume everything else will work out fine. The modern-day history of Africa has shown that transition to independence is difficult whether that independence was granted through the stroke of a pen or won through bloodshed.

But the important thing is that something can be done. The president and his spokesmen should cease always apologizing for the apartheid government in public while insisting that any critical remarks be issued anonymously. They should at a minimum speak out.

Reagan would be the first to say the war on poverty did not end poverty. But it helped, and an entire generation of blacks is thankful for that. Now Reagan has the opportunity once again to convince black Americans that he is the president of all the people, that America can stand up for them as it has for others and that the American dream knows no color, just right and wrong.