The five-year term of one of the governing quintet that sets policy at the Securities and Echanges Commission expired last Monday, and the delay by the White House in filling this key post already is causing concern on Capitol Hill and at the commission - and raising hopes on Wall Street.

Leader of the securities industry have told the White House they would like President Carter to nominate a stockbroker, or at least an industry representative, to fill the vacancy. On the Hill and at the SEC, this prospect is viewed as a serious threat to the delicate balance of the commission. Moreover, there is serious concern over the introduction of politics into an agency that, in its day-to-day operations, at least,has prided itself as being above all that.

But what most troubles this group is that the administration simply has not reappointed the incumbent, John Evans, for another five-year term. Known to some staffers as "the conscience of the commission," Evans, 46, is almost unviversally admired within the SEC.

However, Evans has pushed for the early establishment of a national market system, which, when finally in operation, will give the regional exchanges a share of the business once, traded exclusively on the New York Stock Exchange. Though an open securities market was mandated by Congress, Evans' outspoken endorsement has earned him enemies on Wall Street.

Evans, who has confirm in his post while he awaits a White House decline, refuses to be interviewed. "Maybe I should be selling mayself," he said, "but I can't do that."

Irving Pollack, a fellow commissioner, belives there should be no question about reappointing Evans. "He's a man of impeccable charater and he is equally bright. If the government is to succeed, it needs men like him," said Pollack.

Another SEC official who asked not to be identified, voiced anger at what he viewed as cruel and cavalier handling of Evans' career by the White House. "They're giving him the L. Patrick Gray treatment - letting him twist slowly in the wind," he said.

The chief spokesman for the Wall Street interests is Robert H.B.Baldwin, the managing partner of Morgan Stanley & Co. in New York who is also chairman of the Securities Industry Association, the main trade group for brokers and investment bankers. "It's no secret that Mr.Evans has disagreed with our point of view for a long period of time," Baldwin said in a telephone interview.

Baldwin said that the SIA is satisified with the two SEC appointments made by Carter since he tool office. Last March, after a long delay, Carter appointed Harold Willims, dean of the UCLA graduate school of management, as chairman. Then, in August, the president appointed Robert Karmel, a New York securities attorney, a commissioner.

Baldwin said the White House has requested suggestions for a successor "in case Mr.Evans is not reappointed."

A leading candidate seems to be John Turner, would be an excellent choice, "said Baldwin, who added that several other persons also are under consideration.

Another candidate is a Dayton, Ohio, attorney named John Sealy, who formerly was secretary of Mead Paper Co. and is currently outside counsel to the company. Sealy reportedly has been supported by Ohio's Republican senators, John Glens and Howard Metzenbaum. A Senate source reports that Carter's old Atlanta friend and adviser, Charles Kirbo, has written the president a letter on Sealy's behalf.

Several powerful representatives and senators have written letters to Carter endorsing Evans' reappointment, according to congressional side. He says they include Senate Banking Committee Chairman William Prexmire D-Wise J & Sens. Edward Brooks (R-Mass.) an Sen. John Sparknian (D-Ala.), both senior members of the banking committee, and Reps. John Moss (D-Calif.) and Bob Eckhardt (D-Tex.), ranking members of committees that oversee SEC operations.

In recent years, the five-member commission has expanded the regulatory clout of the SEC over the affairs of corporations and the securities industry. Both business and government observers agree that the commission is going to be tackling even more important issues in the next several years, which is why the Evans' post is considered so important by all sides.

The commissioners meet most days of the week to consider whether to take action on memos submitted by staff members. For example, if the enforcement division wants to instigate a formal investigation of a corporation, it must first get approval of the majority of the commission members. Often the staff will seek a major shift in SEC policy - for example, what corporations must disclose to stockholders. In these cases, the commission will hold open hearings before reaching a decision.

In about 90 percent of the cases, the commission is able to reach a unanimous decision, according to Pollack. Sometimes the staff is ordered back to the drawing board to produce another memo.

The commission, as it is now composed, is a farily diverse mix of talents. Williams, 50, was chairman of Norton Simon Inc. before going to UCLA. Philip Loomis Jr., 63, practiced law before joining the SEC in 1954. He has been a commissioner since 1971. Pollack, 60, once headed the enforcement division of the SEC Known as an extremely principled, tough regulator, Pollack was elevated to the commission in the 1973 in the aftermath of the G. Bradford Cook scandal. Cook, who was briefly chairman, resigned after it was revealed that he sought to affect the SEC investigation of fugitive financier Robert Vesco who had contributed $200,000 to the Nixon re-elction campaign.

Karmel, the firs female commissioner, is said by some critics to be too much of an advocate for Wall Street interest, which she once served as an attorney in the New York firm of Rogers & Wells. Finally, Evans is an economist who taught at the University of Utah before becoming an economic assistant to Sen. Wallace Bennett (R. Utah) in 1964. He went from Capitol Hill directly to his commission job in 1973.

The party in the White House normally holds the majority on the commission. Because Loomis and Evans are Republicans, the White House can be expected to choose a Republican if it decides to replace Evans.

The SIA's Baldwin, apparently taking a potshot at Evans' teaching background, said the challenges facing the SEC demand that the new commissioner "look for practical, not academic, solutions."

Indeed, the lst of subjects suggests that the whole structure of American finance is going to be scrutinized by the five commissioners. Among other things, they will review the role of boards of directors, examine the accountability of accountants, investigate the booming options market, enforce the recently passed laws against payoffs and kickbacks by U.S. corporations abroad, seek solutions to capital raising problems by corporations and, of course, move the securities industry toward the national market system.

With so many economic questions at stake, it is hardly surprising that the industry wants John Evans' seat.