The result of the study was a recommendation that the SEC adopt a uniform fiduciary rule for all brokers that is "no less stringent" than the fiduciary standard governing RIAs that evolved from the 1940 Investment Advisors Act.

But language in Section 913 of Dodd-Frank, dubbed the Wall Street Reform and Consumer Protection Act, may prevent the SEC from adopting a fiduciary standard that would outlaw brokers' commissions, prevent brokers from selling proprietary products, or require brokers to provide ongoing fiduciary standards of care and loyalty.

Those restrictions could lead to an "ill-advised SEC rule" that "could be worse than no rule at all," said Fleming in his report to Congress.

"For the Commission, it will be especially challenging to reconcile what appear to be contradictory mandates under Section 913 of the Dodd-Frank Act—to develop a standard for broker-dealers no less stringent than the existing standard for investment advisers, while accom­modating sales-based compensation and the sale of proprietary products or limited product lines," he explained.

And preventing the SEC from requiring brokers to provide an ongoing duty of care and loyalty could lead to a fiduciary rule that is less "rigorous" than advisor obligations under the Employee Retirement Income Security Act, said Fleming.

Fleming warned lawmakers that that could not only dilute the existing fiduciary standard but also potentially create further investor confusion.

Last March, SEC Chair Mary Jo White publicly called for the implementation of a uniform fiduciary standard, though she cautioned that the rule-making process would be long and arduous.

In May, Chair White announced that Andrew Donohue, the former Director of the SEC's Division of Investment Management, would rejoin the SEC as the Chief of Staff to help draft a fiduciary rule.

The announcement came as two SEC commissioners, Luis Aguilar, who has voiced support for a uniform fiduciary rule, and Daniel Gallagher, a vocal opponent of the DOL's proposal, are scheduled to leave the agency.

Fleming's admonitions to Congress come just weeks before the close of the comment period on the Department of Labor's proposed conflict-of-interest rule, which imposes strict compensation disclosure requirements on brokers acting as advisors to IRA accounts and some 401(k) plans.

Stakeholders in the broker-dealer industry have repeatedly voiced support for a uniform fiduciary standard, but argue that the proposal offered by the DOL is unworkable.

Some opponents of the DOL proposal have called on the SEC to take the lead in the rule making process.

In 2013, Rep. Ann Wagner, R-Missouri, introduced the Retail Investor Protection Act, which would require a SEC rule to be developed before the DOL's rule.

It was reintroduced last February, and referred to the House Subcommittee on Health, Employment, Labor and Pensions at the end of April, after the DOL released its rule.

At a recent symposium hosted by the Insured Retirement Institute, a trade group representing broker-dealers, insurance companies, and asset managers, Troy Paredes, a former SEC commissioner, said he would prefer to see the SEC become the lead agency in establishing a new fiduciary rule, according to reporting in ThinkAdvisor.

But proponents of the DOL's rule say the SEC's rulemaking process could take years, which is far too long for consumers to wait.

"It would be profoundly misguided on legal, policy and pragmatic grounds for the DOL to have to wait for the SEC," said Steve Hall, a securities specialist at Better Markets, a non-profit that lobbies for market reforms and a vocal supporter of the DOL's rule.

At a retirement event recently hosted at the Brookings Institution, Secretary of Labor Thomas Perez reiterated the agency's commitment to posting a new regulation before the end of President Obama's second term, calling the issue a top priority at DOL.

The DOL has already considered comments on the proposal from stakeholders, which are helping regulators "sharpen" their thinking on how to make its proposal, particularly the Best Interest Contact Exemptions, workable, said Perez.

In recent testimony before the House subcommittee on Health, Employment, Labor and Pensions, Perez defended the DOL's proposal against the strains of some lawmakers, who questioned the approximations of investor losses used to rationalize a new fiduciary rule, and whether or not Perez and the DOL adequately consulted with the SEC in the rulemaking process.

"It's a confusing world for people that want to get advice," said Perez, referring to the split fiduciary and suitability standards that now govern the marketplace.

In creating a uniform standard, the proposal will make saving for retirement easier for individuals, and provide small-employer sponsors of retirement plans greater clarity, countered Perez.

But even some Democrats questioned the complexity of the DOL's proposal and the additional burdens it would place on advisors to retirement investors.

Jared Polis, D-Colorado, the ranking member on the subcommittee, voiced concerns over the DOL's proposed Best Interest Contract Exemptions, which would allow brokers to make commissions on investments they recommend, but require extensive cost disclosures, including one, five and 10 year projections of the cost in fees to investors on the products.

"Is that additional burden worth it," asked Polis, who also wondered if such a complicated contract would only lead to investor confusion.

Perez said the DOL would continue to weigh stakeholders' concerns, in order to "operationalize" a workable fiduciary standard from the proposal.