Signatories included the Competitive Enterprise Institute, Americans for Tax Reform, Tea Party Patriots, and the Log Cabin Republicans.

"We urge you to freeze funding in any spending bill for the Department of Labor's (DOL) proposed fiduciary rule until the DOL withdraws such rule," adds the letter.

They claim the DOL doesn't have the authority to write such a comprehensive rule that would "greatly restrict investment choices for 401(k)s, individual retirement accounts (IRAs) and other saving vehicles."

The DOL does of course have jurisdiction over 401(k) plans, as granted under the Employer Retirement Income Security Act.

But opponents of the DOL's rule have argued its efforts to regulate how advisors to Individual Retirement Accounts are compensated constitutes a breach of its jurisdiction, a case that has even been made in retirement legislation proposed last year by Sen. Orrin Hatch, R-Utah.

Others have speculated that that question will be at the heart of legal challenges against the rule when or if it is finalized.

The DOL is currently considering thousands of comment letters from industry and stakeholders relative to the proposal's specifics, which were solicited after a four-day open public hearing in August.

Labor Secretary Thomas Perez and other Labor officials have repeatedly promised substantial operational changes to the proposal.

Opponents say such substantial changes would have to again be reviewed by industry, and are calling on the DOL and the Obama administration to withdraw the rule and repropose a new rule.

The DOL has hinted that the proposal's controversial Best Interest Contract Exemption, which would require extensive disclosure requirements on the sales of commission-based products, will be tweaked to make the provision more operational.

Republican lawmakers recently wrote Sec. Perez, asking him to release a more specific view of just how the rule is being changed, and placed an October 21 deadline for doing so. It has since passed.

Perez recently told ThinkAdvisor that releasing information on the changes before the agency has made a thorough review of the comments would be "irresponsible."

The latest letter from conservative advocacy groups noted that 96 House Democrats recently signed a letter that originated from the office of Rep. Gwen Moore, D-Wisconsin, enumerating several areas of concern over the proposal.

But Rep. Moore and other Democrats who signed that letter later voiced their support for the DOL's efforts.

"We urge Congress to do everything it can, in spending bills and otherwise, to defeat the DOL's destructive fiduciary rule," the letter from conservative advocates concluded.