WASHINGTON (AP) _ William R. McLucas, new head of the Securities and Exchange Commission's enforcement division, has been described by associates as a low-key, down-to- earth guy. Others say he's an intense lawman, savvy in stuff they don't teach in law school.

But most agree that when SEC Chairman Richard C. Breeden tapped McLucas, a 12-year SEC veteran, for what is widely perceived as the second most important job in the agency, it was a vote of confidence in the SEC staff.

''The world sort of saw this as an in-house race between McLucas and several other senior staffers at the SEC,'' said John Coffee, a professor of securities law at Columbia University.

Picking an outsider ''would have had a demoralizing effect on the staff,'' Coffee said.

''It's commendable that people are promoted within the ranks when they deserve it, and from what I've heard about him - he deserves it,'' said Paul Curran, a prominent New York defense attorney.

Breeden, only on the job since October himself, announced his selection of McLucas, 39, on the day after Christmas - ending months of speculation about who would succeed Gary Lynch as Wall Street's top cop.

Lynch, who headed the enforcement division for four years before leaving public service last summer, led the SEC's campaign against insider trading and other abuses by Ivan Boesky and other Wall Street professionals.

The choice of McLucas seemed to please almost everyone, from federal prosecutors to securities industry lawyers - who are themselves mostly former government lawyers.

''He's a really smart and extremely personable guy,'' said Alan Cohen, head of the securities fraud unit of the U.S. attorney's office in Manhattan.

''We were glad to see Chairman Breeden appoint someone from the staff who is familiar with the issues,'' said Susan E. Bryant, administrator of the Oklahoma Department of Securities, a state-level version of the SEC.

''I do not think you could have had a more impressive choice,'' said Harvey Pitt, a Washington lawyer who represented Boesky when the takeover stock speculator was nabbed in the insider trading scandals.

Pitt, a former SEC lawyer himself who has known McLucas for years, said the new SEC enforcement chief is personable but ''he can be very tough, very forceful and energetic.''

Even Arthur Mathews, a Washington securities lawyer who had been touting a member of his firm, Robert McCaw, for the enforcement post, said McLucas was a good choice.

''I don't think anybody inside the SEC would've compared to McCaw, but once the chairman chose to go inside rather than outside, I think McLucas was the best choice,'' said Mathews.

A native of western Pennsylvania, McLucas graduated from Pennsylvania State University in 1972 with honors in political science and a Phi Beta Kappa key.

He attended Temple University's School of Law in Philadelphia where he got his degree in 1975.

After two years as an attorney with the Federal Home Loan Bank Board, McLucas joined the SEC's enforcement division in 1977 and rose from staff attorney to branch chief, assistant director and finally one of three associate directors under Lynch.

While one of his in-house competitors, associate director John Sturc, made a reputation handling the SEC's cases against invesment banking house Drexel Burnham Lambert Inc. and Boesky, McLucas led the agency's cases against former Singer Corp. chairman Paul Bilzerian, California brokerage founder Boyd Jefferies and Rooney Pace, a defunct brokerage.

''He's had his fingers in the bread and butter cases that don't get the headlines like insider trading and parking,'' said Mathews, who represented Bilzerian.

When people describe McLucas, they use words like ''quiet,'' ''confident'' and ''compassionate.''

But Lynch, McLucas's predecessor, said he knows that such descriptions of incoming office holders can get excessive.

''I was described in so many newspaper stories as 'soft-spoken' I thought I should start screaming,'' said Lynch, now in private practice in New York.

''I wouldn't describe Bill as low-key at all. I think he's a very intense person, as a matter of fact. He certainly has a lot of common sense.''

If McLucas is down-to-earth, Lynch said it's because he has ''the knowledge you don't get in law school.''

McLucas, who is said to smoke a pipe occasionally and follow the football fortunes of his alma mater, declined to be interviewed last week, stating it was too soon for him to be talking to reporters.

''My preference is to get my sleeves rolled up and get the cases going and then give interviews,'' he said.