In theory, the Supreme Court is insulated from politics. But that isn't stopping the opposing sides on the abortion issue from mounting intense lobbying campaigns as the justices prepare to hear a controversial Missouri case that could end the country's current era of legal abortions.

The Supreme Court is expected to hear the appeal, Webster vs. Reproductive Health Services, in late April.Meanwhile, both the self-anointed "pro-life" and "pro-choice" movements have prepared dozens of "friends of the court" legal briefs.

Beyond that, both sides also have begun to launch a series of calculated moves - marches, sit-ins and newspaper and magazine ads as well as letter-writing blitzes aimed at the Supreme Court, the Attorney General and the White House - that will build in intensity over the next six weeks.

The underlying design is to put pressure on the Supreme Court: the activists are convinced the court's decision can be swayed by public opinion.

"As far as we're concerned there's no tomorrow. Everybody knows the question is about to be called," said Sheri O'Dell, vice president of the National Organization for Women, which favors abortion on demand.

"Our people want the Supreme Court and President Bush, who's pushing the court on this issue, to know just how they feel," she said. "We are co-sponsoring what promises to be a massive march in Washington on April 9th and I won't be surprised if a quarter of a million demonstrators show up; our people are furious."

O'Dell, who is coordinating NOW's role in the march, said she thinks a big turnout won't go unnoticed at the court.

"Whatever the decision is," she said, "it will be couched in legal terms but, in fact, it is a political question that, like Prohibition, ultimately will be decided by public opinion."

Judie Brown, president of the American Life League which opposes legalized abortions, said she, too, thinks political considerations will weigh heavily in the Supreme Court's decision.

"Public opinion means everything to the court," said Brown, who says her organization includes 200,000 families around the country. "We feel all forms of expression will impact on the court."

The American Life League's most expensive form of expression will be a series of full-page advertisements in USA Today, The Washington Post and The Washington Times. The ads will run during the week before the opposition's April march and will cost $150,000.

"They will include just-released information we've taken from the file of the late Justice William O. Douglas during the debate on Roe vs. Wade," she said. "It's clear from these files that a small but very vocal minority was able to influence the decision which made abortion legal in this country."

In Roe vs. Wade, the court held that while a woman's right to privacy gives her the right to have an abortion, the state may regulate or prohibit abortions once the fetus is viable.

Since the 1973 decision, there have been relatively few third-trimester abortions even though that same court, in Doe vs. Bolton, said the mother's life or health should be an overriding factor in the decision of whether to permit an abortion even after the fetus is viable.

In case to be in April, the Missouri Legislature passed a 1986 statute banning abortions at public hospitals or any other public facilities unless the woman's life is in danger. The act, whose preamble states that viability begins at conception, also forbids public funds from being used for abortions or for abortion counseling except when the mother's life is threatened.

The statute was challenged immediately by two abortion clinics, Reproductive Health Services of St. Louis and Planned Parenthood in Kansas City.

In March 1987, a federal district judge in Kansas City declared the major sections of the statute to be unconstitutional infringements of Roe vs. Wade. In July 1987, the 8th U.S. Circuit Court of Appeals upheld the bulk of the lower court's decision.

Last year, William Webster, Missouri's attorney general, asked the Supreme Court to review the case. He also asked the court, if it should side with the appeals courts, to then consider whether Roe vs. Wade itself is constitutional.

The Supreme Court announced in January it would consider the case, setting off a flurry of planning and activity on both sides of the issue.

"We've already raised most of the $150,000 for our upcoming newspaper ads," Brown said, adding that $150,000 is an exceptionally large expenditure for the American Life League.

The National Abortion Rights Action League already has spent more than $500,000 on newspaper advertising. The full-page ads, which feature a coat hanger, a reminder of the days of back-alley abortions, first ran on Jan. 22.

"We're running them in some of the major newspapers across the country," said Kate Michelman, NARAL's executive director. "We're also doing radio and TV ads and making ourselves available to the media for interviews and talk shows and the like."

So far, she said, there has been a tremendous response to the ads, which ask for pledges of support.

"We've had more than 30,000 responses to the January ads," she said. "The ads cost $300,000 and we took in $350,000."

Michelman said while NARAL sees a drawn-out fight over the abortion issue, she feels the Supreme Court can be influenced.

"There's no doubt we're in trouble, given the changes in the court since Roe vs. Wade," she said. "But the Supreme Court doesn't act in a vacuum; it is sensitive to the political and social climate of the country."

Because of this, said David Andrews, acting president of the Planned Parenthood Federation of America, his group is trying to mobilize mainstream Americans.

"There's no question this is a political issue and, while the Supreme Court can't be lobbied directly, it can influenced by a great outpouring of voices," he said.

Barbara MacGear of Operation Rescue, a Binghamton, N.Y.-based, pro-life group which has helped stage protests at abortion clinics around the country, said, "We plan keeping with our agenda" of demonstrations and civil disobedience.

"Frankly," she added, "I don't know whether the Supreme Court can be swayed by what happens in the streets but we're watching closely. What we're really hoping for, though, is a right-to-life amendment to the U.S. Constitution."

Douglas Johnson, legal director of the National Right-To-Life Committee, acknowledged that the court's acceptance of the Missouri case and the resulting news media focus have "provided an unusual forum."

He said he has been "kind of amused" by some of the efforts to get the Supreme Court's attention.

"In our view," he said, "these efforts are unlikely to influence the case. Our legal brief certainly isn't going to talk about the Harris Poll, it talks about the Constitution."

Then, he added, "but Roe vs. Wade was a political decision, an act of judicial lawmaking."