Paul Dowling is proof of the unpredictable nature of the American voter and the complexity of American issues.

There he was, the proverbial man in the street, seated on a park bench in the center of the most conservative city of this most conservative state, a young, unemployed shoe factory worker who has suffered the pain of economic hardship since being laid off two years ago. And what does this man who voted for Ronald Reagan in 1980 consider New Hampshire's most important issue?

Air pollution.

"A lot of the counties have fallen behind in air-pollution control and probably noise pollution," he said. "Someone should really see that they are meeting the standards . . . . This is an 87 percent foliaged state, but you come up here and choke on the air pollution."

His reply is not as surprising as it first appears.

Concern about the environment has become a potent political issue here, cutting across all elements in the state and seemingly destined to affect the outcome of next winter's highly publicized traditional springboard, the presidential primary.

New Hampshire's message to presidential aspirants already crisscrossing this state of rugged mountains to the north and urban sprawl to the south is clear: ignore the environment at your peril.

Already, Democratic candidates are vying with each other to demonstrate which has the best environmental proposals.

If President Reagan runs here again, he too must reckon with these intensifying concerns. Politicians of both parties agree that, despite his undeniable personal popularity in this state, Reagan must be careful how he deals with environmental records of Interior Secretary James G. Watt, actions of former Environmental Protection Agency administrator Anne M. Burford and the overall environmental response of his administration--if he does so at all.

These are among the findings in a week of interviews with citizens throughout New Hampshire by four Washington Post reporters and a Post poll of 1,189 Manchester area residents.

No longer is the environment an issue of primary appeal only to affluent citizens who can afford to indulge in popular causes, or to organized environmental activists. Here, the environment also carries weight in the working wards of Manchester. It is a basic bread-and-butter issue, with good reason.

The people of New Hampshire have been gripped by new fears about their air and water. One can scarcely turn on a television set or pick up a newspaper anywhere in the state without encountering another report of a serious environmental problem. Even those who believe that the media have overplayed the issue go on to describe neighbors' fears about finding leaking chemical wastes buried in woods behind their homes.

Toxic waste dumps are leaching chemicals into water tables around the state, contaminating wells and drinking water. In some places, citizens have had to boil their water.

Discovery of new, dangerous toxic waste sites, coupled with the feeling that New Hampshire has been used as a dumping ground by trucks crossing the state line at night, have increased citizens' concern. Democratic presidential candidate Walter F. Mondale, who has made eight political trips to the state in recent months, addressed these emotions by saying New Hampshire should be a breeding ground for America's future, not a dumping ground for its past.

Acid rain, not long ago an esoteric subject of limited public awareness, has become a major concern in virtually every corner of New Hampshire.

Farmers tell of ponds where no fish swim or of lakes that are dying, the result of sulfur emissions borne from midwestern factories to New Hampshire by western winds and deposited over the landscape in rain. Foresters report damage to woods and mountainsides from the same acidic compounds.

Since tourism is the state's second largest industry, after manufacturing, with hunting and fishing among the major attractions luring vacationers, damage to forests and streams poses a direct economic threat to New Hampshire's livelihood.

A vivid demonstration of how powerful this issue has become occurred last March. In what amounted to a referendum on acid rain, New Hampshire citizens voted in 197 of 221 town meetings in favor of halving sulfur emissions to limit acid rain. The vote stunned environmental groups uncertain whether citizens would even consider such a proposal, much less approve it overwhelmingly.

Even the major economic problem confronting citizens across the state--the rising cost of energy reflected in utility bills--is directly related to an environmental issue, nuclear power.

The nuclear plant at Seabrook that looms over the Hampton beaches on the state's Atlantic coast, long a rallying point for demonstrators opposed to such facilities, has become a symbol of something other than nuclear safety. It is a monument to nuclear costs.

Electric rates could triple or quadruple, by some estimates, when Seabrook becomes operational in 1986.

"In an economy like New Hampshire's, we couldn't absorb that kind of a shock," said Rep. Norman E. D'Amours (D), whose 1st Congressional District stretches from Manchester to the coast.

"I think New Hampshire now recognizes Seabrook is overboard," said Michael Power, director of the state's Office of Vacation and Travel in Concord. "It's way too expensive. It's going to be a hell of a price. It's just an expensive albatross."

Such concerns add up to a growing awareness of environmental problems in a state that has not experienced the high unemployment of other areas during the recession. These collective environmental concerns are forming a powerful political issue that will figure in the debates of 1984 and beyond.

No single incident has heightened sensitivities about environmental issues. But many people, subjected to a barrage of reports about new problems in New Hampshire and surrounding states, now feel, as one put it, "that things are declining. They have a sense that it is getting worse and that it has to be stopped."

Of these concerns, D'Amours said:

"It's extremely potent politically. Look what's happening to people in the rest of the New Hampshire congressional delegation. People who traditionally have been very insensitive to the environment suddenly are involving themselves very deeply in these issues.

"You hear these stories about trucks driving up New Hampshire's roads in the dead of the night with their valves open and just trailing all this toxic stuff behind them. That's what did it. That's what made people recognize you can be an environmentalist without being a tweety-bird watcher.

"They know that environmentalism means your health and your life. That's what's made it the issue."

Residents are aware that New Hampshire's quality of life is threatened and seems likely to be for many years. They know the problem is not transitory or trendy, and they wonder how best to deal with it.

This flinty state's historically conservative citizens have not been radicalized politically.

They were brought up to distrust government at all levels, especially Washington, and they have been unwilling to raise taxes to provide funds for public services considered basic in other areas. Perhaps because of their background, people active in environmental issues often note that they are not environmentalists.

"To me, that's a dirty word," said activist Hartley Bailey, who discovered hazardous wastes in ground water near his Kingston-area home in southern New Hampshire. "So I'd prefer not to be known as an environmentalist. That means a do-gooder, a do-nothing, always in somebody else's hair."

His wife, Martha, put it differently: "They're interested in birds and bees, flowers and fauna and all that. We are not environmentalists. But I do care what people drink."

With others, the Baileys are undergoing a change that affects personal attitudes and political actions.

"It became a shock to find out how dirty our state is," said John Sherburne, looking back after one term in the state House of Representatives where, as a Republican, he served on the Agriculture and Environment Committee.

A Reagan voter, he, too, was no political activist. He retired to a farm in Deerfield after an Air Force career that began with fighter-pilot missions in World War II and ended with Vietnam. He expressed a common sense of discovering the severity of his state's environmental problems.

"Suddenly, we've got all these hazardous waste dumps," Sherburne said. " . . . It was interesting this morning. A young lady who's working on her PhD at Harvard, her husband lives here. She commutes. So I stopped her and told her you were coming by.

So we were discussing this and she said: ' . . . The other night when we were talking about hazardous-waste sites, I didn't worry too much about the one down in Epping, 20 miles away. But when you told me there's one five miles away, in Northwood, I was shocked.' "

Sherburne paused and said: "What's the difference between 20 miles and five miles? The point is that about 60 of them are in the state suddenly. No one knew they were there . . . . How far does it have to go before you really consider it a crisis?"