When people on Capitol Hill and at the Federal Trade Commission speculate about who will succeed Commissioner David A. Clanton after his term expires Sept. 25, the name Terry Calvani comes up most frequently now.

Calvani, a law professor at Vanderbilt University Law School since 1974, is a recognized expert on antitrust law. He has co-authored a book, "An Economic Analysis and Antitrust Law," and served on committees of the American Bar Association's Antitrust Section.

He acknowledged that the Reagan administration has contacted him about a possible nomination. The White House declined comment.

"If I were nominated, I would accept it," he said. He refused to comment on major issues facing the commission.

James Rill, a Washington lawyer who worked with Calvani on an ABA committee, said, "He would bring a balance between antitrust legal scholarship and practical antitrust litigating experience as a lawyer." Besides teaching, Calvani practices law with North, Haskell, Slaughter, Young & Lewis of Birmingham.

Although Calvani generally is understood to be FTC Chairman James C. Miller III's choice, his nomination is not certain, some FTC officials say. Mike Hammond, an aide to Sen. Jesse Helms (R-N.C.) is pursuing the job. According to the newsletter Antitrust & Trade Regulation Report, Hammond has the backing of the U.S. Chamber of Commerce and many senators. * * *

CREDIT PRACTICES . . . House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and the chairmen of key House panels sent Miller a letter last Friday, urging the FTC to expedite the necessary paper work to promulgate a new rule regulating credit practices.

In July, the commission unanimously approved the substance of the rule, which prohibits the most abusive credit practices. But, because of the impending departure of Clanton--the swing vote on the FTC for the past year--some commission staffers expressed concern that the technical background papers would not be approved in time. If the papers are not accepted, it is possible that some of the rule's provisions, or all of them, could be changed.

The letter from the congressmen was initiated by Rep. James J. Florio (D-N.J.), who chairs the subcommittee in charge of overseeing the FTC.

"The main reason we are concerned is the potential . . . of the procedure being modified once the composition of the commission changes," Florio said.

Florio said that Miller assured him in a telephone conversation that the FTC was acting in good faith on the credit-practices rule and the used-car rule, a controversial regulation that requires dealers to disclose known defects and that the FTC has just voted to reexamine. "But I think it's important the Congress . . . have their views known," Florio added.

Among the 15 members who signed the letter were Reps. Fernand J. St Germain (D-R.I.), chairman of the Banking, Finance and Urban Affairs Committee; Peter W. Rodino Jr. (D-N.J.), chairman of the Judiciary Committee; and Parren J. Mitchell (D-Md.), chairman of the Small Business Committee.

Rep. John D. Dingell (D-Mich.), chairman of the Energy and Commerce Committee, sent a similar letter to the FTC this week, an aide to Florio said.

All five FTC commissioners were out of town yesterday, but commission staffers said they appreciated the support expressed in the letter.

"Rules are traditionally attacked as soon as they see the light of day and, for a change, some members wanted the first reaction to be positive," one official said.

The official added, however, that Sen. Jake Garn (R-Utah), chairman of the Senate Banking, Housing and Urban Affairs Committee, sent the commission a letter urging that the rule not be promulgated.