President Reagan announced yesterday that he plans to nominate Terry Calvani, a 36-year-old Vanderbilt University law professor, to the Federal Trade Commission, a move that is expected to finally give conservatives a clear-cut majority on the commission.

Calvani, if confirmed by the Senate for the seven-year term, will replace David A. Clanton in the $67,200-a-year FTC commissioner's post. A moderate Republican originally appointed by President Ford, Clanton has in recent years been the swing vote on a five-member agency otherwise sharply split between two conservative Reagan appointees, including chairman James C. Miller III, and two liberal holdovers appointed by former president Carter, including former chairman Michael Pertschuk.

Calvani, a graduate of the University of New Mexico and Cornell Law School, is expected to tip the balance, although not radically so, FTC officials and outside legal scholars said yesterday. As an antitrust expert who has chaired several American Bar Association committees and the author of numerous publications in professional journals, Calvani has repeatedly argued for an "economic" approach to antitrust law that takes into account the play of market forces. Yet while unquestionably conservative, Calvani is not "overly ideological," said one law professor familiar with his work.

More significant, Calvani will be the first lawyer in the conservative camp at the FTC, given that both Miller and his ally, George W. Douglas, are economists. "Miller was pushing for this . . . because he wanted an antitrust lawyer to help him face some of the heat he's been getting from the Hill and from some of us on the commission," said Pertschuk.

Pertschuk also said, however, that Calvani is "a reasonably well-known quantity. Given the range of possible disasters, he's probably not the worst we could have expected."

In a brief statement issued through his spokeswoman, Miller said only that Calvani has "outstanding" credentials and "I'm sure he would make an excellent commissioner."

More ideologically conservative GOP senators and the U.S. Chamber of Commerce had pushed for the nomination of Michael Hammond, an aide to Sen. Jesse Helms (R-N.C.), over Calvani.