Two months ago, as the campaign to defeat Supreme Court nominee Robert H. Bork was getting off the ground, Durwood Zaelke of the Sierra Club Legal Defense Fund expected to sit out the fight, as he had last year during the bitter struggle over confirmation of William H. Rehnquist as chief justice.

When organizers of the anti-Bork effort called to invite Zaelke to meetings, he recalled, "I said I thought it sounded very interesting, but I've got cases to bring. I think I sent one college intern to one meeting, trying to be polite."

This week Zaelke's group, along with the 416,000-member Sierra Club, the Natural Resources Defense Council, Friends of the Earth, Greenpeace and other environmental organizations, broke with their traditional neutrality on judicial nominations and came out against confirmation of Bork.

The switch occurred, Zaelke said, because Sierra Club members "prodded us into doing it," calling the office with inquiries about Bork. After studying decisions by the jurist and concluding that he would be hostile to environmental claims, Zaelke said, "we decided to take some action. We had watched Reagan appoint 300-plus judges. And this was it. This was the final straw."

The environmental groups' decision to weigh into the battle against Bork reflects an ever-broadening array of groups -- many of which have rarely or never before take positions on judicial nominees -- opposed to the federal appeals court judge. It is part of a carefully orchestrated strategy to keep up a drumbeat of opposition to confirmation, to persuade wavering senators that there is widespread concern about Bork and to counter the truckloads of letters and thousands of telephone calls supporting Bork from "right-to-life" and conservative groups.

Arrayed on the pro-Bork side are the Knights of Columbus, the Fraternal Order of Police, the National Federation of Republican Women and the Southern Baptist Convention Public Affairs Committee, according to a list released last week by the White House. But groups supporting Bork have been less visible in Washington. Instead, right-to-life, religious and conservative organizations have concentrated on direct-mail and telephone campaigns, with some success.

At the office of Sen. Arlen Specter (R-Pa.) -- a key vote on the Senate Judiciary Committee -- where staffers answer the phone, "Sen. Specter's office; are you calling about the Bork nomination?" letters from constituents were running 25,373 for Bork and 10,414 against as of Tuesday.

But in the battle of Washington news conferences, the Bork opponents are clearly ahead. On Wednesday, for example, groups representing the disabled, including the Associated for Retarded Citizens, the National Mental Health Association, the Epilepsy Foundation of America and the United Cerebral Palsy Associations, lined up against Bork. Coming up soon: the National Council of Churches, whose 31 member churches represent 40 million people; senior citizens' groups, and small-business organizations.

"I think you're going to see a lot more activity and opposition expressed by a vast number of organizations," said Nan Aron of the Alliance for Justice. "There will be at least a press conference a week from a major segment, a major group of organizations."

The health organizations are described by Leonard Rubenstein of the Mental Health Law Project as "mainstream groups which cannot be characterized as liberal interest groups by anyone's definition." The involvement of such groups is an attempt to blunt the charge by Bork supporters that the opposition to his confirmation comes only from "extremist" groups.

With Bork having finished his testimony before the committee and the confirmation battle having entered what Aron calls "chapter two," the effort on both sides to generate grass-roots sentiment has intensified and widened its focus from the 14 members of the Judiciary Committee to the full Senate.

"A lot of what it is incumbent on us to do now is convey to the senators not so directly involved" the importance of the nomination, according to Melanne Verveer of People for the American Way.

In the meantime, she said, "The committee work continues to go on -- the preparation of testimony, tracking what Bork was says, for example, and being able to spot immediately the change in his record."

"At this point, the strategy is to go as broadly as we can to reach as many senators as possible, not limit it to those on the Judiciary Committee," Aron said. "All these Senate offices want information: 'How's it going, where are the problems, what's the response?' So we are getting large numbers of calls from Senate offices asking for summaries" of Bork's testimony.

Even before Bork began testifying last week, the American Civil Liberties Union (ACLU) held two nights of briefings, attended by staff from 40 Senate offices, on the Bork nomination.

Bork opponents have emphasized what Sen. Patrick J. Leahy (D-Vt.) described as Bork's "confirmation conversion" -- the issue of whether Bork shifted his positions in such areas as the coverage of the equal-protection clause and the scope of the First Amendment in order to increase his chances of being confirmed.

However, ACLU lobbyist Jerry Berman said, "We have to make the case that the so-called new Judge Bork is not very different in degree from the old Judge Bork. If it's not an issue of credibility for the senators, it's an issue of predictability: knowing who is Robert Bork. Is he an intentionalist or is he a new person? We think that on the whole his judicial philosophy remains the same."

Conservative groups supporting Bork, for their part, have stressed his moderate testimony on women's rights, the First Amendment and racial issues. At the same time, they have sought to paint his opponents as extremists.

"We need to criticize our enemies quite aggressively," said Patrick McGuigan of Coalitions for America, which plans a radio ad criticizing Bork's foes. "It's very interesting to judge a man by his enemies."

"I think for the most part the conservatives are attacking the opponents of Bork and tying the senators who are going to be opposing Bork into the opposition," said conservative direct-mail expert Richard Viguerie. "So the senators who oppose Bork are going to be identified now and forevermore with Planned Parenthood, People for the American Way, the ACLU, Jesse Jackson, all these other wild people out there."