In June, Campbell authored a paper on behalf of the Chamber that was highly critical of the DOL's proposed rule, which would insist a fiduciary standard of care on most advisors to IRAs and workplace retirement plans.

Specifically, the paper, which was widely reported on, claimed the proposal would put onerous requirements on advisors to small-business plan sponsors, forcing them to exit that space or ultimately pass more costs on to sponsors and participants.

That was but one grievance Campbell raised in testimony he gave Tuesday.

"Our members, especially small businesses, have been very clear: the rule wouldn't help us, it would hurt us," said Campbell.

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"Unfortunately, the proposal's regulatory burden and choice limitations fall hardest on those very small businesses that already have the most difficult time offering retirement plans," he added.

Campbell called the rule "discriminatory" against small businesses, due to the provision that requires advisors to plans with fewer than 100 participants to act as fiduciaries. The rule exempts advisors to larger plans from being fiduciaries, on the grounds that those sponsors have existing internal fiduciary protections.

"Our members believe that the proposal, with its massive new compliance and legal liability costs for advisors, will make it infeasible for a portion of those advisors to serve the small plan marketplace," he said, citing 2011 DOL data that showed a lack of access to advice and resulting investment mistakes cost savers $114 billion in 2010.

That cost "is several times greater than the estimated cost motivating this proposal," noted Campbell.

Among the Chamber's laundry list of issues with the proposal, Campbell said the removal of a "mutual understanding" requirement between participants and advisors will result in greater confusion as to whether the information being exchanged is fiduciary advice or not.

He called the retention of that concept "critical."

Hauser, whose academic bearing has so far yielded little, if any frustration with witnesses critical of the rule, took issue with the idea that the DOL should require participants and advisors to have a mutual understanding of the proposal's fiduciary requirements.

Requiring a mutual agreement to trigger fiduciary status would be "riddled with the potential for abuse," said Hauser, suggesting the DOL might not be willing to budge on the matter.

"Mutuality is inherent in any contract," countered Campbell. "It ought to be adopted here as well."

Hauser also expressed frustration over Campbell's and other witnesses' repeated claims that the rule clouds the distinction between advice and education.

"We keep hearing that a casual conversation will trigger fiduciary status but that is not the case," he said in defending the clarity of the proposal, which Hauser argued specifically states that information would have to include a recommendation to be considered fiduciary in nature.

In his testimony, Campbell said Chamber members are also concerned that the proposal prohibits a "vital" education tool by defining allocation models that reference a plan's existing investment options as fiduciary advice.

"Removing a proven tool that has not lent itself to abuse requires more than a general concern that people might mistake it for advice," said Campbell.

In the question-and-answer portion of the panel, which was dominated by the back-and-forth between the two, Campbell made references to the overall ambiguities in the proposal.

Hauser also took issue with that suggestion.

"There are multiple levels of ambiguity in a process like this. Some have been legitimately identified, and some are talking points in an effort to try to defeat the rule," said Hauser.

"You can count on our continuing to work with people to identify ambiguities," he added.