BOSTON -- Kate Michelman was in the middle of her talk, a stock speech on how important the 1988 presidential campaign would be for reproductive rights, when everything changed. Someone handed her a note saying that Supreme Court Justice Lewis Powell had resigned.

Suddenly, Michelman, head of the National Abortion Rights Action League, was looking straight into her own worst scenario, the one penned in the most hyperbolic of the league's newsletters. Ronald Reagan, enemy of women's right to abortion, now had the chance to appoint the justice who could wipe out that right.

By late the same night, Michelman had heard from dozens of supporters, including her three grown daughters, who were, she says, ''all shocked at the reality they faced.'' Despite the fact that the overwhelming majority of Americans -- including 72 percent of Republicans -- continue to support choice, ''it could happen that we wouldn't have legal abortion.''

Powell had not been the perfect pro-choice justice. The man in the middle had cast the swing vote against Medicaid funding for poor women and had devised a compromise that left standing parental consent laws for minors. But when the chips were down, he had voted with the shrinking majority that continued to support the reasoning of the 1973 Roe v. Wade decision.

What will happen now? When Attorney General Edwin Meese recently told an audience that the administration has no litmus test for its court candidate, they all laughed. There is a test and it's anti-abortion. The nominee to replace Powell, Appeals Court Judge Robert Bork, passes with flying colors: he has called the court's use of the privacy argument in Roe ''judicial imperialism.''

The scenarios for a new court that will emerge from a Reagan appointment range from bad to catastrophic. The good news is that no one believes the Supreme Court would define the fetus as a human being. The current justices are split 4-4 on abortion as a constitutional right, but the four dissenters don't all agree with each other.

It is possible that a new court would merely (this is now relative) erode the right to abortion, by limiting access. It's possible it would merely uphold an increasingly long and obstructive list of state regulations governing consent forms, medical procedures, rules that limit when and under what conditions women can have abortions.

It's also possible that the new court would overturn Roe altogether, finding no constitutional guarantee for abortion. That would throw the issue back to the states.

There is no record of a democratic country reinstituting a ban on abortion. Socialist countries (Rumania) or fascist countries (Nazi Germany) have done so. History, like public opinion, is in the direction of allowing women the choice of abortion.

But as Kathleen Sullivan, a constitutional law professor at Harvard, warns: ''We could go back to a patchwork where there are free states and criminal states.'' We could go back to the pre-Roe days when the right to a legal abortion hinged on whether a woman could afford a plane ticket to New York or California. Abortion would be illegal in some places, merely impossible in others.

I don't want to write as if this were a fait accompli. Justices can surprise us and their presidents. More to the point, any nominee must be confirmed by the Senate; one out of every five is rejected.

Civil-rights and women's-rights groups are gearing up for a major fight. ''The next appointee stands between us and a radically different life in this country,'' says Michelman.

Nevertheless, it's gloomily believed that the president has found a winning Supreme Court candidate, and that candidate has passed the big test. So, says Prof. Sullivan, ''We can't declare the day of legal abortion is over, but it's going to be in for some rough times.'' And those times begin right now.