Some members of Congress, when they are strapped for money, step up their speaking engagements. Others, such as Rep. Marty Russo (D-Ill.), do a little moonlighting.

Russo is earning $20,000 per year as counsel to a Chicago law firm with clients whose interests could fall within the jurisdiction of the House Ways and Means Committee, of which he is a member.

A Russo aide and a partner in the law firm said that Russo without exception will avoid cases involved with pending legislation and will, as the law requires, not practice before federal courts or agencies. He will offer advice for the most part, they say, and will not discuss federal issues.

His part-time employment is legal, and Russo is not the only member of Congress to perform legal services for which he receives income while in office.

But many of the roughly 25 other members of Congress who receive income from law firms have retained ties with firms they were with before being elected, or are receiving fees for cases they worked on before their elections. Russo, who has been in the House for 10 years and has not practiced law for nine years, began working for Coffield Ungaretti Harris & Slavin last month.

The House ethics code says that working for a law firm while in public office "may raise ethical and conflict of interest problems beyond those involved in other outside employment undertakings." And the rules of professional conduct of the American Bar Association say lawyers in public office should not let their names be associated with a law firm without "actively and regularly practicing with the firm."

The practice is prohibited in the Senate, where members cannot be affiliated with a law firm, have their names used by a firm or work for compensation during office hours.

Russo sought a legal position with Coffield Ungaretti because he felt that it was a better way to earn outside income than by earning honoraria through speaking engagements, said Bob Macari, Russo's administrative assistant. Russo's former aide and former campaign finance chairman, Joseph A. Cari Jr., is a partner in Coffield Ungaretti and helped arrange his appointment as counsel to the firm.

"What this is is an attempt by someone who's very tired of the honorarium system," Macari said. "In this situation, we can say there's no conflict under the law," where, with honoraria, legislators can earn fees from groups interested in pending legislation.

"This connection is being stretched to the 'Nth' degree," Macari said, adding that Russo has researched and will abide by all House ethics strictures in his practice. "Last year he made $20,000 in outside earnings and this year he's making $20,000 in outside earnings. What's the difference?"

Russo will not accept fees for speaking engagements now that he has this job, which will occupy him between four and 10 hours a week, Macari said. He would not be able to take many in any case: His $20,000 salary brings him close to the $22,467 maximum amount he can earn in outside income under House rules this year. Members of the House are allowed to earn outside income of up to 30 percent of their $74,891.63 annual salaries.

Alton B. Harris, a partner in Russo's new firm, said it specializes in real estate, litigation, mergers and acquisitions. It also has as clients several commodities firms and commodities traders, he said. The firm helps commodities dealers in their registration with federal agencies, in federal enforcement cases and in personal tax and financial planning work, he said.

"He will not do anything that will conceivably disqualify him from his duties," Harris said. "He certainly wouldn't be dealing in tax issues that affect those people."

Russo is not the only member of Congress to receive income from law firms. Rep. Joseph P. Addabbo (D-N.Y.) earned about $8,000 last year, according to his office, from consulting with the firm he founded, Addabbo & DeSena. Rep. Mario Biaggi (D-N.Y.) made $10,400 in consulting fees in 1983 from his former firm, Biaggi & Ehrlich.

Whether legislator-lawyers should practice has been a hot topic since Daniel Webster represented the Bank of the United States in 41 cases before the Supreme Court while a senator in the 1830s. Currently, the House code of ethics raises several possible conflicts that could arise from a member of Congress practicing law.

The interest of a client could diverge from the broader national interest, it says, and the practice could divert a member's time from his official duties.

"Faced with such ethical considerations and potential conflicts of interest, many members of Congress have voluntarily refrained from practicing law," according to the manual prepared by the Committee on Standards of Official Conduct.

The manual also points out that the American Bar Association's model rules of professional conduct put a lawyer-legislator in something of a no-win sitation. If he practices too diligently, he is in danger spending too much time away from the duties of his office.

If he merely lends his name to the firm and gives occasional advice, he could run into the ABA rule specifying that a lawyer's name can be affiliated with a firm only if he practices there on a regular basis. The ABA code also cautions against possible conflicts of interest.

Senate Rule 37 prohibits members from "affiliating with a firm, partnership, association or corporation for the purposes of providing professional services for compensation." Nor can a member practice any profession for compensation.