CHICAGO (AP) _ David Sturtevant Ruder, President Reagan's choice for chairman of the Securities and Exchange Commission, is a respected legal scholar who will bring strong administrative skills to his new job, associates said Wednesday.

And the professor and former dean of Northwestern University's law school ''can be tough, too,'' said Arthur H. Laun Jr., a longtime friend and managing partner at Quarles & Brady, the Milwaukee law firm where Ruder began his career.

''I don't think they have to worry about his watchdog image,'' Laun said in a telephone interview.

After meeting with Reagan in Washington, Ruder said he would continue the SEC's strong enforcement policy against insider trading if the Senate confirms his appointment.

''I think the commission's on the right track,'' he said of recent SEC involvement in the insider-trading scandal on Wall Street.

Ruder, 58, proved himself a skilled administrator from 1977 to 1985 as dean of Northwestern's law school, which he ''revitalized'' while helping organize two annual institutes for corporate and securities lawyers, Laun said.

''I'm delighted'' with the nomination, said Robert Bennett, who became dean after Ruder returned to research and teaching.

''I would say David has been perfectly schooled for this job and I think he will do a tremendous job,'' Bennett said.

Ruder, described by associates as a moderately liberal Republican, has never held a government position, said Northwestern spokesman Chuck Loebbaka.

However, he has taught courses in SEC enforcement, insider trading and tender offers, and has written more than 40 articles on corporate securities matters, Loebbaka said.

He also practiced law for four years at Quarles & Brady and five years at Schiff, Hardin & Waite, a large, prestigious Chicago firm with a Washington branch.

''He's a super person and he'll be great for the job,'' said Stuart Goodman, a senior partner at Schiff, Hardin, where Ruder worked part time from 1971 to 1974 and then full time for two years after taking a leave of absence from Northwestern.

''He's written extensively, he's spoken at seminars across the country, so he's very well known and very respected by securities lawyers,'' Goodman said.

Reagan said in a statement Wednesday that Ruder is the best choice ''to continue the fine work of Chairman John Shad in the area of insider trading enforcement.''

Shad is leaving the SEC to become ambassador to the Netherlands.

Born in Wausau, Wis., Ruder graduated with honors from Williams College in Williamstown, Mass., in 1951, and received his law degree with honors from the University of Wisconsin in 1957.

He was an associate at Quarles & Brady from 1957 to 1961 before accepting a faculty position at Northwestern.

From 1972 to 1980, Ruder was a consultant to the American Law Institute Federal Securities Code Project, which produced a draft Federal Securities Code combining six major federal securities laws into one.

In 1978, he was appointed by the New York Stock Exchange to a legal advisory committee on which he served four years.

Ruder is married to attorney Susan Frankel and lives in north suburban Highland Park. He has four children and two stepchildren.