SIXTEEN years after the Roe v. Wade decision, the days of legal abortion in the U.S. are nearly over. The 1973 Supreme Court decision guaranteeing the women the right to an abortion is likely to be substantially undermined--though not reversed--by the Rehnquist court.

The social changes produced by recriminalizing abortion doubtless would be enormous. But the most notable difference in an America without Roe could well be the transformation of American politics. If the high court overturns Roe, two distinct possibilities arise: Republicans could realize their ambition to capture a congressional majority, or they could lose their hold on the presidency.

The Roe decision has been in jeopardy since the Reagan administration committed itself to appointing antiabortion justices. This year, the court agreed to hear the case of Webster v. Reproductive Health Services, in which the state of Missouri challenged the Roe decision. In 1986, the last time the Supreme Court considered the issue, it reaffirmed Roe by a 5-4 vote. But since then, Justice Lewis F. Powell, who voted with the majority, has been replaced by Justice Anthony Kennedy, whom many believe opposes Roe.

Justice Harry Blackmun, author of the majority opinion in the original Roe decision, predicts that Roe will likely "go down the drain this term." Even if it doesn't, Blackmun and his liberal brethren, Justices Thurgood Marshall and William Brennan, are teetering on the brink of retirement. Another Republican-appointed justice would seal Roe's fate.

If the court does overturn Roe, it will probably not ban abortions entirely, but give discretion to the states, some of which will prohibit abortion. Idaho already has a law on the books requiring the governor to ban abortions if the Supreme Court overturns Roe. Most states will place some restrictions on abortions without prohibiting them entirely, while a few will maintain the legal status quo.

FOR women with unwanted pregnancies, America without Roe will seem remarkably like America before abortion was legalized. Those with money will be able to find safe abortions. Those who don't will have either unwanted children or back-alley abortions.

The social consequences of overturning Roe would be lamentable. But the political consequences would ultimately be just as significant because abortion would become a political rather than a judicial issue. In an America without Roe, the Republicans, the party that pursued the anti-abortion cause, will either flourish or perish.

The first, less likely scenario is that allowing state legislatures to determine abortion laws will give local politics an unprecedented ideological character. William F. Buckley predicts that the Republicans, armed with the emotional abortion issue, will gain a majority in most state legislatures.

These bodies will redraw congressional districts after the 1990 census, and Republican-controlled states could draw boundaries that favor the GOP. Since the current districts are widely thought to be gerrymandered in favor of Democrats, such a shift could give the Republicans a majority in the U.S. House of Representatives for the first time since the 1950s.

But this conservative fantasy is unlikely. Those 43-45 states inclined to restrict abortions would probably do so right away, making abortion laws a fait accompli, not a political issue.

A more likely result of overturning Roe would be far less palatable to the GOP. An end to legal abortion could spell the end of Republican hegemony in presidential elections.

The electoral majority that Republicans enjoyed in the last three presidential races rests on an unstable alliance between two antagonistic groups. The first, the fundamentalist Right, is a vocal minority that gets much recognition, but has little substantive power. The second is a group I call MBA Republicans--young, educated, economically conservative voters who are uncomfortable with the New Right's social agenda.

Republicans have been able to hold this untenable alliance together by paying lip-service to the social agenda of the Right without taking any serious action on it. MBA Republicans don't care for the fundamentalist Right, but they still see no reason to fear them.

Overturning Roe would change all that. Up until now, the fundamentalist Right has resembled Oscar Wilde's description of George Bernard Shaw: they have no enemies in the Republican party, but they're disliked intensely by all of their friends. If MBA Republicans see that the party is actually carrying out the Right's social agenda, they will defect en masse.

The political importance of abortion should not be underestimated. According to one poll, 44 percent of Republicans oppose a ban on abortions. Opposition among Independents and Democrats who vote Republican is even stronger. Moreover, 78 percent of college-educated voters and 68 percent of Westerners oppose a ban. These are not groups that Republicans can afford to lose, especially since the GOP's electoral lock on the West gives it the presidency.