Staff members of the Federal Trade Commission were at odds yesterday as the agency began the final stage of ruling on an eight-year-old proposal to police the consumer credit industry.

The split is between the deregulation-minded aides appointed by the Reagan administration's FTC chairman, James C. Miller III, who oppose the plan, and the long-time staff professionals, who believe the commission should protect Americans from what they see as unfair credit practices and who accused their superiors of using "newly raised theories" to discredit the proposed regulation.

The five-member commission yesterday heard the first of two days of final arguments from industry foes and consumer fans of the proposed regulation. The FTC will decide the issue next Monday.

Miller and newly named Commissioner John Douglas are expected to agree with economists on the staff who say the cost of policing consumer credit outweighs the benefits, while Commissioners Patricia P. Bailey and Michael Pertschuk are likely to support the proposed regulation.

The stance of David A. Clanton, the fifth commissioner, whose term is nearing an end, remains a question mark.

The proposed rule is aimed at helping consumers cope with what the commission staff called a wide variety of abuses by the credit industry.

The rule's provisions include making sure collateral is sold at the best possible price to meet a debt, barring contract provisions that allow lenders to pick up a debtor's salary without going to court, limiting contacts by creditors with such third parties as employers or relatives, restricting provisions forcing debtors to pay lenders' attorney fees, and prohibiting customers from waiving the right to protect some of the property from seizure to satisfy a debt.

"We see important social benefits in these rules" that are hard to quantify in any cost-benefit analysis, Dwight Golann of the Massachusetts Attorney General's Office told the commission yesterday.

Timothy J. Muris, who was director of the FTC's Bureau of Consumer Protection while the proposal was being prepared there and is now director of the Bureau of Competition, and Wendy Lee Gramm, director of the Bureau of Economics, took the other side in opposing the rule. Carol T. Crawford, the new director of consumer protection, agreed with them.

"This rule would probably cost consumers more than it is worth" and hurt its intended beneficiaries, high-risk borrowers who will find credit more difficult to obtain, Muris said.

"Everyone will find credit more costly," he added.

Staff members who worked on the rule, however, disagreed strongly with Muris, Crawford and Gramm. The opposition of those three officials relies mainly on "opinions, hypotheses and assumptions--both stated and unstated--which find little or no factual support in the record or this proceeding," wrote FTC attorneys Christopher W. Keller and Norman E. Oliver of the Bureau of Consumer Protection.

"The commission does not need to engage in speculation on newly raised theories to resolve the questions at issue in this proceeding," they told the commission.