A coalition of consumer and labor groups has mounted an unprecedented challenge in the General Assembly to the reappointment of a Virginia corporation commissioner who they charge consistently has favored utility interests in rate-making cases.

The coalition is seeking to oust Commissioner Preston T. Shannon, who they say has been the key advocate for utilities such as the Virginia Electric and Power Co. in rate decisions made by the powerful three-member commission. Shannon, they say, once even joked to Vepco representatives that they were "pikers" for not asking for higher rate increases than they had requested in one case.

"Shannon's record has been pretty clearly against consumers in Virginia," said Maureen Kennedy, a consumer activist from Charlottesville who has been helping to coordinate the campaign against Shannon, a 56-year-old former lawyer for the Chesapeake and Ohio Railway. "And with his experience in energy issues, the other commissioners tend to follow him."

No incumbent corporation commissioner ever has been denied reappointment by the assembly, which elects commissioners to six-year terms. Kennedy and other consumer spokesmen today predicted they already have enough votes to oust Shannon and replace him with former Del. Erwin S. Solomon when the election of the two-term commissioner comes before the legislature next week. Solomon is a Bath County lawyer who has represented consumer groups before the commission and was an unsuccessful candidate for state attorney general in l977 and l981.

Some of the legislators involved in the movement to remove Shannon as commissioner are more cautious about the vote count: "I think we've got a shot at it but I'm not saying we've got it," said Sen. Virgil H. Goode Jr. (D-Franklin). "It's up in the air."

The move to oust Shannon is the latest attempt to reform the commission, a semi-autonomous agency that regulates not only public utilities but also state-chartered banks, savings and loans, securities, trucking and railroads. With a $20.6-million budget and a staff of more than 500, the three commissioners are considered among the most powerful of Virginia officials, although they are not subject to general election and seldom have been challenged. The commissioners, who also have the powers of state circuit court judges, each are paid $55,980 a year.

Among the recent commission decisions that have aroused the ire of consumer groups was a ruling last summer that allowed Vepco to charge customers for the continuing cost of constructing power plants. Consumer groups say Shannon played the principal role in persuading fellow commissioners Junie L. Bradsaw and Thomas P. Harwood Jr. to overturn a previous policy prohibiting those costs to be passed on to customers until the plants were operational.

Shannon today refused to comment on the campaign to replace him. But commission spokesman John Daffron dismissed the criticisms as unfair. The commissioners, he said, "sit as umpires and sometimes nobody likes the umpire . . . Nobody likes these high utility rates. But sometimes the drum gets beaten because somebody can see a little political hay to make out of it."

Shannon faces a public hearing Monday before a legislative committee at which consumers are expected to make the case against him. His fate won't be decided until early next month when the Democratic caucuses of the Virginia House and Senate meet--probably behind closed doors--to vote on his reelection.