Seventy-five placard-carrying senior citizens and union members demonstrated at the Federal Energy Regulatory Commission yesterday to demand that its chairman disqualify himself from all natural-gas pricing proceedings.

The group, organized by the Citizens/Labor Energy Coalition, contended that Chairman C.M. (Mike) Butler III, had made numerous "inappropriate and prejudicial statements" about his intention to use the administrative process to raise gas prices.

Butler was in New York on business yesterday, out of earshot of the demonstrators, who chanted, "Hey Mike, what do you say, close that backdoor right away," outside FERC headquarters at 825 N. Capitol St.

A spokesman for the chairman said he would meet with the group's leaders and respond to their demands when he returns.

The "backdoor" reference was to what the consumer group believes is a Reagan administration strategy to sidestep a confrontation on accelerated gas price decontrol with Congress, where resistance is heavy, but to achieve the same result "backdoor" through its power over the price regulatory process FERC administers. The group sees Butler as the administration's point man on the effort.

FERC is considering a proposal to raise by 50 percent the price ceiling on gas found between 10,000 and 15,000 feet--so-called intermediate deep gas.

Butler, a Reagan appointee, also plans to ask the four-member commission next month to consider increases in the price of "old gas"--gas discovered and drilled prior to 1978. If these are adopted, the price of all natural gas would rise by about a third.

The group demanded hearings around the country on both issues.

They met briefly yesterday with two of the four FERC commissioners, Georgiana Sheldon and John D. Hughes, the lone Democrat, but received little besides expressions of concern. Hughes said the matter of whether Butler should recuse himself would be left up to Butler.