Nuclear industry digs in its heels. Nuclear power was in trouble long before Chernobyl. Dozens of US nuclear projects have been scuttled, and public confidence has plunged. But the industry is fighting back.

Despite its many problems, nuclear power is not fading into the sunset. Indeed, the promoters of the industry are gearing up for a battle likely to stretch well into the next century.

Rebuilding public support is their primary goal. Pro-nuclear groups have mounted national publicity campaigns emphasizing nuclear power's contribution to the ``energy mix'' of the country. Nuclear plants provide 18 percent of the nation's electricity.

Money is also being poured into local campaigns, such as the fight against tomorrow's referendum in Maine which would shut down that state's only nuclear power plant. (Story, Page 8.)

And last, the nuclear industry is pushing for changes that will make it easier to build and license plants in the future.

``The nuclear industry clearly believes it will come back,'' so it's working at setting up the framework for that now, says John Ahearne, a former chairman of the Nuclear Regulatory Commission and vice-president of Resources for the Future, a Washington think tank.

Nuclear advocates confirm this. ``I have no doubt that we're going to go back to nuclear energy - the only question is when,'' says Harry Finger, president of the pro-nuclear US Council for Energy Awareness.

Mr. Finger's council is an example of the industry's high-pressure push to get its message across. Recently reorganized, the council now combines the public relations activities of what were previously divided between two nuclear-power related advocacy groups. The group spends about $7 million a year on national advertising, which includes television commercials warning about the dangers of over-reliance on imported oil.

``The industry is streamlining its structure in Washington in order to magnify its impact,'' says Scott Denman, director of the Safe Energy Communication Council, an antinuclear group.

Mr. Denman says this has already helped the industry. For example, in the congressional fight over renewal of the Price-Anderson Act - the federal law that limits the liability of utilities that operate nuclear power plants - the industry now speaks with a more unified voice.

The pro-nuclear lobby already has considerable clout with the Reagan administration, which strongly supports nuclear power. The administration has encouraged simplification of licensing procedures, extension of Price-Anderson, and the development of standardized designs for nuclear reactors.

The industry won a major victory last week when the Nuclear Regulatory Commission voted to allow federal officials to approve emergency evacuation plans for the regions surrounding nuclear power plants, even when state and local authorities refuse to cooperate. Such state and local opposition has blocked the opening of two plants - Seabrook in New Hampshire and Shoreham in New York.

But selling nuclear power to the public won't be easy. Harris polls conducted since 1975 show that the number of people opposed to building new nuclear plants has grown over the last decade, peaking at nearly 80 percent just after Chernobyl. When asked how they felt about having a plant built within five miles of their homes, the people were even more adamant.

Indeed, some analysts insist nuclear power has already entered its twilight. ``These days you can't even site a landfill, much less something that's going to have radioactive material in it,'' says Christopher Flavin, an energy analyst with the Washington-based Worldwatch Institute.

For the average voter, nuclear power is not a hot issue. But it can be made into one, especially when the issue strikes close to home. Take the referendum to close down Maine Yankee nuclear power plant. In that case, antinuclear activists - who failed twice before in bids to close the plant - have latched onto the issue of nuclear waste. The Department of Energy is selecting underground storage sites for waste from the nation's nuclear power plants, and Maine is a candidate. Voters are being told that closing Maine's only nuclear plant will reduce the chances that a dump will be built there.

Besides environmental and safety concerns, the public is also sensitive to the changing economics of nuclear power. Across the country, consumer groups are balking at paying for costly and unnecessary power plants. Many facilities were planned in the 1960s and '80s - based on projections for future energy demand that never materialized.

Nuclear power was sold as the low-cost alternative. And many early plants were able to deliver on the promise.

But in recent years, nuclear projects have had cost overruns of legendary proportions. Seabrook was supposed to cost $2 billion when ground was broken in 1976. Instead, it will cost $5 billion - possibly forcing the utility that owns most of it into bankruptcy.

The key issue is who should pay for investments that seemed reasonable in the past, but not any longer.

In Chicago, the local utility is asking for a 27 percent rate increase to pay for three nuclear plants that are finished but won't be needed for at least five years. Consumer advocates want to split the cost between utility investors and ratepayers.

``We're not talking about bankrupting the utility,'' says Susan Steward, director of the Citizens Utility Board of Illinois, a Ralph Nader-inspired consumer group. ``But we do think utilities should be forced to face the cold realities of the marketplace.''

In some cases, this means swallowing a hefty loss, even if this means getting rid of the problem plant entirely. The Zimmer plant in Ohio, for instance, was abandoned in 1984 after construction problems were found that would have been too expensive to fix. It's now being converted to burn coal - while the utilities that own it are suing the architectural and engineering firm for $1.2 billion.

Tomorrow: Nuclear plants as a financial ball and chain.