Opponents of pension regulations announced recently by the Treasury Department are expected to place a hold on the nomination of John W. Snow to be Treasury secretary unless the rules are rescinded.

The hold will be placed by Sen. Tom Harkin (D-Iowa), sources said. Holds are a Senate custom that allows a single senator to prevent a bill, nomination or other measure from reaching the floor.

The nomination of Snow, who is chairman and chief executive of Richmond-based CSX Corp., had appeared to be heading for a confirmation vote as soon as today.

The rules involved cover "cash balance" pensions, which are being adopted by many large companies to save money or because cash balance plans better suit the needs of the workforce, or both. Cash balance plans provide better benefits for younger and shorter-tenured workers but can penalize older workers.

Opponents of cash balance plans argue that their impact on older workers violates federal age discrimination laws, and a number have filed lawsuits against employers, including International Business Machines Corp., that have converted traditional pensions to cash balance plans.

During the Clinton administration, the Treasury Department and Internal Revenue Service placed a moratorium on cash balance conversions. However, late last year the Treasury Department proposed regulations that would say specifically that cash balance plans are not discriminatory. It is those rules that opponents want to see withdrawn.