Big speculators could face curbs on wheeling and dealing in commodity markets after the Commodity Futures Trading Commission Thursday advanced a plan to cap large positions, but internal dissent could delay final adoption of the plan for months or longer.

An important swing vote on the five-member U.S. futures regulator expressed skepticism about whether curbs on their trades would prevent a large run-up in prices.

“With such a lack of concrete evidence, my fear is that, at best, position limits are a cure for a disease that does not exist or at worst, a placebo for one that does,” Commissioner Michael Dunn said at the opening of the hearing.

The plan, adopted in a 4-1 vote to allow it to move to the public comment stage, would attempt to curb positions investors can hold in commodity markets, with the aim of preventing large players from controlling the market.

Consumers and some lawmakers say it would prevent a run-up in prices, which hit record highs in 2008 in oil and many food staples.

But the surprisingly strong talk by Dunn, a Democrat, will add fuel to a debate about the role banks and funds play in energy, metals and agricultural markets, whose prices are soaring again.

His skepticism means a final rule on aggregate limits could be delayed, or even shelved, said Daniel Waldman, a partner with law firm Arnold & Porter, and a former general counsel at the CFTC.

“I thought it was an indication that there is a chance that the Commission may never have the votes to put these aggregate position limits in place,” Waldman said.