The political hallmark of James K. Asselstine was stamped on the first day of his first presidentially appointed post.

Asselstine had been chosen by President Reagan for a seat on the Nuclear Regulatory Commission, and within a day of his Senate confirmation on the Thursday evening of May 13, 1982, he began receiving anxious telephone calls from the White House and NRC Chairman Nunzio J. Palladino. They wanted him to take his oath of office as soon as possible -- preferably that weekend -- so that he could vote at a commission meeting to come right after the swearing-in ceremony.

The meeting was to reconsider an administration request to begin site preparation for a controversial demonstration breeder reactor in Tennessee. The commission had rejected the proposal two months earlier, with three appointees of President Jimmy Carter outvoting two members named by Reagan.

In Asselstine, the administration apparently thought it had shifted the majority on the five-member panel. Then the Republican counsel for a Senate subcommittee dominated by GOP conservatives, Asselstine seemed like an amenable replacement for one of the departing Carter appointees who had opposed the site preparation request. The other two opponents were traveling outside Washington on that crucial spring weekend.

Asselstine protested the rush as "unseemly," he now says, but finally yielded to Palladino and took his oath on Monday. While one of the opponents was still out of town, a meeting was called. But the newest member voted against the measure.

"I was sworn in at 11 in the morning," Asselstine recalls. "The meeting was scheduled for two o'clock. I spent those three hours writing my first set of dissenting views."

It has become a familiar experience for Asselstine, frequently the sole dissenter on a panel that Reagan has stocked with conservatives of his own political stripe. As an advocate of strong government regulation of the nation's 100 atomic plants, Asselstine sharply differs from a commission majority that has shown a greater inclination to accommodate concerns of the nuclear power industry.

The very mention of Asselstine's name sparks the most extreme comment in a field where extreme views are common. Depending on the observer, he is a safety fanatic or voice of reason, a truculent critic of fellow commissioners or a constructive devil's advocate, a technical naif or a skilled synthesizer of complex issues.

He calls himself "the moderate on the commission. The other four are the mavericks. They've shifted to the right. When I talk to regulators in other countries, I find myself in the mainstream."

At home, however, Asselstine tries to overcome his numerical disadvantage with the power of rhetoric. In his written dissents and congressional testimony, he is the rare Washington official who publicly denounces his colleagues and challenges the conventional wisdom -- which, in the nuclear arena, amounts to the proposition that atomic plants pose an "acceptable" risk to the public health and safety.

According to some of his more pungent critiques:

*The 1985 decision to allow the restart of Three Mile Island's undamaged reactor, six years after the Pennsylvania plant suffered the worst U.S. nuclear accident, reveals a commission majority "satisfied with Band-Aid, short-term fixes" as a solution to the utility's management problems.

*The commission majority's decision in 1983 to extend its deadline for a New York plant's emergency evacuation plan "makes a mockery" of its own regulations.

*The majority failed to provide "even the most rudimentary explanation of or justification for" a 1985 decree that endorsed the safety of U.S. plants and concluded that there is no urgency to fortify them against the risk of severe accidents. Asselstine called the commission ruling "a complete failure."

Perhaps his harshest dissent came in the majority's decision last September requiring the staff to prove "substantial" health and safety benefits before it could impose safety improvements on existing plants -- a ruling, he said, in which the commission "continues down its inexorable path toward nonregulation of the nuclear industry."

"I can think of no other instance in which a regulatory agency has been so eager to stymie its own ability to carry out its responsibilities," he said.

Asselstine differs from his four colleagues -- none of whom agreed to be interviewed for this article -- in background as well as philosophy. At 38, he is the youngest commissioner. He is the only lawyer on a panel that includes a retired vice admiral of the nuclear Navy, a nuclear physicist and a former businessman whose company sold reactor containment liners. Chairman Palladino, whose five-year term ends July 1, has worked in both academia and the nuclear industry.

Asselstine has spent his whole career at the intersection of nuclear and legal affairs. He started at the NRC in 1973 as a legal intern and staff attorney for its Atomic Safety and Licensing Appeal Panel. After two years he moved to Capitol Hill as assistant counsel for the Joint Committee on Atomic Energy, and then back to the NRC as a staff attorney for its regulations division. More recently, he was a counsel to the Senate Environment and Public Works Committee and its nuclear regulation subcommittee.

A source at the commission said Asselstine's slashing style alienates other members and isolates him. He limits his effectiveness, said the source, by taking rigid stances on issues instead of negotiating to move the final decision closer to his position.

"He'll put himself in the most extreme position even where there's wriggle room in which he could compromise," the source said.

"Sometimes, Jim is suggesting that the other commissioners don't do a conscientious job," said Bart Cowan, a Pittsburgh lawyer who represents the nuclear vendor Westinghouse. "They're almost ad hominem attacks. It doesn't further the process by criticizing the motivation of other commissioners or casting aspersions."

Asselstine demurs. From his first day of dissent in 1982, he said in a recent interview, other commissioners have refused to accommodate his views. He agrees that he is isolated, but adds, "I'm painted in the corner by the others, not by myself. They take the positon that, 'If Jim doesn't like it, that's too bad.' "

His sometimes cutting style, he said, is intended to draw public attention to commission decisions that he believes to be wrong. "I'm left with little recourse," he said.

A former commission member said, "A lot of things happened on the personal level that radicalized Jim."

For all the criticism of his style, Asselstine insists that his dissent is focused on pressing matters of substance.

"The fundamental difference between Asselstine and the others is that he takes seriously the proposition that his constituency is the public, not the industry," said Ellyn Weiss, general counsel for the Union of Concerned Scientists.

But Carl Walske, president of the nuclear industry trade group Atomic Industrial Forum, called Asselstine a "perfectionist" in an industry that achieves acceptable levels of imperfection. He said that "because human beings are involved, because machines are involved, this demand for perfection isn't possible."

"If you never want to make a mistake, you're always going to play Mr. Negative," said Walske. "You're never going to make a decision."

To the charge of perfectionism, Asselstine pleads no contest.

"If nuclear power is to survive in this country, you have to have good performance from the industry and a credible regulator," he said. "That's something the commission has lost sight of. It seems to go out of its way to undermine its own credibility . . . .

"The public has to have confidence that the commission is functioning as a competent regulator that has the public's protection as its first priority. If you lose that, the whole endeavor is in jeopardy."

Asselstine's term expires June 30, 1987. Asked what he might do next, he laughs. "I just don't know. I would welcome the opportunity to stay on the commission, because I think there is much work to be done. But I suspect my ability to do so is -- " he pauses, seeking, this once, to pull a punch, " -- fairly limited."