Four state legislatures—California, Illinois, Massachusetts and Washington—have enacted legislation. The two other state proposals the GAO looked at—West Virginia’s and Maryland’s—have yet to pass initiatives into law.

All of the proposals face regulatory uncertainty, most notably in the form of the Employee Retirement Income Security Act’s preemption clause, that will continue if action is not taken by Congress, the DOL and the Department of Treasury, the report said.

Absent clarity at the federal level, the prospect of legal challenges is likely to slow the implementation of state-run or state-facilitated plans.

California and Maryland’s plan would require employers with five or more employees to automatically enroll workers into an IRA-based program administered at the state level.

Washington plans to create a state-facilitated marketplace for businesses with fewer than 100 workers that would opt-in to a savings plan. West Virginia’s plan would also be voluntary, as would Massachusetts’, but it would only be for small not-for-profit employers.

The GAO’s report lays out a strong statistical argument for the state initiatives.

One study the report draws on from the Current Population Survey is the participation rate in workplace plans in the private sector has declined to 43 percent since the 1990s, when it was about half.

Another data set shows only 39 percent of private sector workers age 21 to 64 participated in workplace plans in 2012.

The GAO also says the vast majority of workers who don’t participate in workplace plans—84 percent—don’t do so because they don’t have access.

About 68 percent said they worked for an employer that did not offer a program, and another 16 percent said they weren’t eligible for the program.

And only 23 percent of workers employed at firms with fewer than 50 employees participate in plans, compared to 60 percent of workers at firms with more than 1,000 employees, according to the GAO.

ERISA’s preemption clause invalidates any and all state laws that relate to employee benefit plans.

Stakeholders in the state initiatives fear their plans could be subject to litigation, and, perhaps worse, expose employers that choose to offer state plans to potential liability.

“Given these implications of uncertainty regarding ERISA preemption, state efforts to expand access to millions of workers and address the retirement savings shortfall may be delayed or deterred,” said the report.

The GAO added that none of the initiatives it reviewed were immune from legal uncertainty caused by ERISA’s preemption provision.

The DOL has already initiated new rulemaking that would address the concerns of state plans’ compliance with ERISA, per the White House’s direction.

The agency plans to post proposed new rules by the end of 2015, the GAO said.