President Reagan is expected to name a former legislative aide to Louisiana Democrat Sen. J. Bennett Johnston Jr. to be the fifth member of the Federal Energy Regulatory Commission.

The nomination of Oliver G. Richard III comes at a time when the FERC is in the middle of a political storm over whether natural gas prices should be raised. And the nomination is likely to be controversial because it will provide a forum for criticism of the commission and because members of Congress from the Northeast and the Midwest have urged that their regions be represented.

Richard is expected to be named to a seat on the commission that must be filled by a Democrat or an independent. No more than three members of the commission may belong to a single party, and there are already three Republican members. The Republicans are Chairman C. M. (Mike) Butler, Anthony Sousa and Georgiana Sheldon.

Richard, who now is a partner in a Lafayette, La., law firm, would replace Matthew Holden Jr., a Democratic member whose service with the FERC ended last September. If Richard is appointed, he would be the third member of the commission from the Southwest. Butler and Commissioner David Hughes are from Texas.

The Northeast-Midwest Congressional Coalition of the House of Representatives wrote Reagan May 3, urging him to appoint a commissioner from a natural-gas-consuming state, "particularly one from a state in the Midwest or Northeast."

The appointment is considered critical by certain consumer groups because the fifth member might be the swing vote on raising natural gas prices. Hughes and Sheldon have adopted a more cautious attitude on that issue than Butler.

"Natural-gas-consuming states are not adequately represented on the commission," according to a letter to Reagan from Reps. James L. Oberstar (D-Minn.), Robert W. Edgard (D-Pa.), Silvio O. Conte (R-Conn.) and Frank Horton (R-N.Y.). "We would be seriously disappointed if the current vacancy were not filled with an appointee who could speak for those states in the Northeast and Midwest in the important deliberations on natural gas that the commission is currently undertaking."