RICHMOND -- When you serve in the General Assembly as long as Franklin Slayton has -- 16 years -- you accumulate a lot of chits. Slayton, who was defeated for reelection last month, is calling his in.

The prospect of landing one of the juiciest political plums in state government, commissioner on the State Corporation Commission, has Slayton salivating. The job pays $78,216 a year, carries the title of judge and would greatly enhance the retirement benefits Slayton has earned as a legislator. (In the judiciary, one year's service counts as 3 1/2 years toward retirement.)

The problem is, there is no vacancy on the three-member commission. Preston C. Shannon, who has served on the SCC since 1972, is up for appointment to a new six-year term and has no intention of retiring.

The SCC is one of the most powerful state agencies in the nation, regulating the banking, insurance, utility and transportation industries, and overseeing more than 120,000 incorporated businesses. It was established in 1902, with popularly elected commissioners, which were eliminated by the Byrd organization in favor of appointees. The commission has 528 employes and an annual budget of $27.2 million.

Its three commissioners are appointed by the legislature, which, in the overwhelmingly Democratic assembly, translates to the Democratic caucuses in each chamber.

Shannon apparently has the backing of the House and, to a lesser degree, the Senate hierarchy.

So the candidacy of 55-year-old Slayton, a soft-spoken Democratic lawyer from South Boston, near the North Carolina border, may become a test of the assembly leadership.

The Senate leadership already lost one recent battle and there is talk of further attempts at democratization of that chamber.

Slayton's supporters, who include Sens. Clive L. DuVal 2nd and Joseph V. Gartlan Jr. of Fairfax and Del. Warren G. Stambaugh of Arlington, say he would bring a more pro-consumer attitude to the SCC.

But in the House, especially, the lineup has all the earmarks of a test of strength by the middle-aged (young would be chronologically stretching it) Turks.

Shannon has the support of Speaker A.L. Philpott of Bassett, Majority Leader Thomas W. Moss Jr. of Norfolk and caucus Chairman Alson H. Smith Jr. of Winchester; Slayton's backers are led by Stambaugh and C. Richard Cranwell of Vinton -- sometimes referred to collectively as "Cranbaugh" -- who see themselves as the next generation of House leaders.

"It's not a test of power," Cranwell protested. "Absolutely not."

So why is he backing an outsider? "We need someone on the commission with the consumers' interest at heart," Cranwell said. Does that mean he believes Shannon is anti-consumer? Not necessarily, said Cranwell, his feet firmly planted on either side of the Roanoke River.

Stambaugh -- who, not surprisingly, prefers the collective moniker "Stamwell" -- adds that "fairly or unfairly, Shannon's record has been characterized as pro-utility and anti-consumer." He described Shannon as "an honest person who does what he thinks is right, but it always seems right to him to go along with the utilities."

Stambaugh said he doesn't pretend to know what kind of a commissioner Slayton would be, noting the futility of congressional attempts to predict how Supreme Court nominees will act as justices. But Slayton "would bring a little more fresh air" to the commission, which Stambaugh said might give consumers "a little more equity."

Shannon, 62, a railroad lawyer before joining the SCC as a staff lawyer in 1968, describes himself as "pro-fair play." He said his record shows that "I have done a lot for the consumers," including originating the heat-sharing program for the poor and getting reduced-rate telephone hookups and service for the underprivileged.

Even if the supporters of Shannon and Slayton manage to choose a winner without having the decision come off as a challenge to the leadership, the 1988 session, which begins Jan. 13, is likely to be turbulent.

Many think this term will be the last for Philpott, who sees fewer of his cronies in the well of the chamber each year he presides. This time around he is sure to miss former Finance Committee chairman Theodore V. Morrison Jr. of Newport News, who did not seek reelection. If Philpott's oilman pal Lewis W. Parker Jr. decides to quit after this term, that may be the straw that sends Philpott back to Bassett.

Over in the Senate, no one suggests that Majority Leader Hunter B. Andrews may retire. In fact, some say, he is still dreaming of being governor, although that vision was partially shattered by his inability to muster a majority of his own party on behalf of his choice for president pro tem.

The challenges to Andrews are likely to continue. Even his position as majority leader is not immune, although further attacks on his power are more likely to be oblique.

Whatever happens, most senators appear committed to making sure neither Andrews nor any of his successors is allowed to consolidate the power held by the late Edward E. Willey of Richmond.

"There's not going to be an Ed Willey, Part II," said one senator.