When Francis M. Mullen Jr. left the security of 20 years with the FBI last year to head the Drug Enforcement Administration, a lot of people thought he was crazy.

The FBI, after all, is more than a job. It's a family and a way of life. Why, his colleagues asked, would a perfectly sane man in the top tier of the bureau's leadership leave to direct a group of agents who often look and act more like dope dealers than federal lawmen?

Some DEA staffers also were less than enthusiastic, complaining that Mullen seemed more like a Boy Scout leader than a drug agent. As Joel Lisker, a former FBI agent who now is chief counsel to the Senate subcommittee on security and terrorism, put it: "There's a joke at the bureau: An FBI agent's idea of going undercover is taking off his tie."

And when the Reagan administration announced last January that it was reorganizing the DEA into a semi-independent agency under the FBI, morale at the drug agency plummeted.

Agents wondered if they would be fired and replaced with FBI agents, whose education and training requirements are tougher than the DEA's in some areas.

Perhaps most of all, the 1,826 agents at the DEA were afraid their agency would simply be swallowed by the FBI, which has more than four times as many agents.

A drug enforcement effort has been in place under one name or another since 1915, and the DEA has been under its current name for the past nine years. It always has been a troubled agency, plagued by corruption charges and often treated like the stepchild of federal law enforcement, shifted from place to place on the bureaucratic charts.

And in the 16 months since FBI Director William H. Webster persuaded Mullen to take its helm, Mullen has not had an easy time.

As the FBI's executive assistant director for investigation, he was criticized for the way the agency handled the Abscam probe and the background check of Labor Secretary Raymond J. Donovan.

Critics say that in Abscam, Mullen allowed FBI agents to target congressmen without sufficient evidence of corruption. In the Donovan case, he was faulted for not promptly passing on to a Senate committee unsubstantiated charges and references to Donovan that had come up in other FBI organized crime investigations.

His nomination has been held up since January while the Senate Labor and Human Resources Committee looks into his role in the Donovan investigation. In the meantime, Mullen is "acting" administrator.

Bud Mullen, as he is known to friends, was born 47 years ago in New London, Conn. His father was in the Coast Guard, and the rest of the family stayed behind in New London while his father was at sea with the Coast Guard.

After high school, Mullen spent four years in the Air Force, then enrolled at Central Connecticut State College in New Britain, got married and began working nights at the New London police department 51 miles away.

He majored in history, planning to become a high school teacher. But he found himself hooked on law enforcement. With dreams of manhunts for the men on the posters in the post office, he joined the FBI in 1962 for $6,995 a year.

Mullen moved quickly through the FBI ranks, tracking kidnapers, bank robbers and other gangsters all over the country. By the time he left the agency last year, he was one of its four top officials.

Coworkers say Mullen is not the typical high-level appointee. When he went to the DEA, he was astounded when he was presented with a car and driver. The driver finally was reassigned when it became clear that Mullen was doing most of the driving.

If Mullen has a fault, colleagues say, it is that he is "naive," and not politically sophisticated. He doesn't generally hold press conferences or try to get his name in the news.

His tastes are simple. He lives in a pleasant -- but not fancy -- subdivision near Wolf Trap Farm Park and drives a Ford Escort subcompact. He's at work by 6:30 a.m. every day, but makes it a point to be home by 7:30 every night for dinner with his family. Friends say that except for official functions, his social life -- what there is of it -- is mostly within FBI and DEA circles.

Outside his work and family, Mullen has three major interests. He runs 4 1/2 miles every day and plays tennis about four times a week with people such as Webster and Attorney General William French Smith. And in his back yard, he has a carefully tended garden still full of tomatoes, peppers and flowers.

In a small, wood-paneled den, Mullen has surrounded himself with the treasures of his life. There are FBI coffee mugs on a bookcase, and scrapbooks full of news clippings from cases he has worked on.

A line of tennis trophies along the top shelf were won by his wife Nancy, an assistant vice president of a Virginia bank. He notes with pride that he taught her to play.

On the walls are framed letters of commendation he received over the years at the FBI, many from Hoover. There is an award won by his son in the army in Germany. In the center of the wall is his wedding picture, hung right beside one of Mullen and Webster.

Mullen is confused and angry that the Senate won't move on his appointment. "If anyone has done anything to go after organized crime in this country, it's me," he said. "There's that inference there that I didn't want to expose organized crime. . . . If anyone has been tougher on organized crime, I'd like to know who it is."

Staff members involved with the Senate Labor Committee say they expect Mullen's nomination will go through eventually, possibly during the post-election session. One said, "There's no evidence that Bud in any way did anything wrong."

Both the attorney general and the FBI director said last week Mullen still has their full support and they are reasonably certain he will be confirmed.

Roger Young, an assistant director of the FBI and a close friend of Mullen, said, "If there were problems, that's all they were--problems of communication, not deception or conspiracy. . . . Bud had never been before a congressional committee to testify about a nominee."

But there's no question that the delay in Mullen's confirmation has had an unsettling effect on the DEA. There have been daily rumors that he will leave. One rumor was that he would return to his FBI post, which Webster still is holding open. Another particularly far-fetched one was that he had decided to run for mayor of New London.

Mullen says he's determined to stay and to try to restore morale at the drug agency.

There have been changes since he took over. The FBI and the DEA now are cooperating in more than 200 drug cases, up from 12 before the merger took place. The FBI is working alone on more than 600 others. Webster says, "The mutual respect between the agencies is going up every day."

The FBI, with larger numbers and specialized training, has been able to help on wiretaps and financial investigations. And the law has been changed to allow the military to spot boats or planes carrying drugs.

It's too early to tell what the changes will mean in terms of cutting into U.S. drug trafficking, but Mullen and Webster feel they can make a difference. "Now they'll have the substance and clout of the FBI behind them," Mullen said.