Tony James and Teresa Ghilarducci are unlikely allies. He is president of Blackstone, the giant private equity firm; she’s a labor economist who has long advocated replacing 401(k)’s with a universal, federally managed saving plan.

But the two have teamed up to push what they are calling Guaranteed Retirement Accounts, a government-sponsored plan that would require participation and contributions from any employer without its own 401(k). They both view the 401(k) defined contribution retirement system as a faulty experiment that covers too few workers, generates inadequate savings and replaces too little income in retirement.

“There’s really no alternative,” Mr. James argues. “It needs to be mandated.”

Maybe so, but the notion of employer mandates has become anathema to Republicans in Congress since enactment of the Affordable Care Act in 2010, which requires employers above a certain size to provide health insurance for their workers. The bitter political battle over the health law killed any prospect for the White House’s mandatory retirement savings proposal — the auto-IRA — which was first proposed in 2009.

“Anything that was a requirement for employers, and especially smaller employers, became incredibly toxic and politically charged,” said David John, senior strategic policy adviser at AARP, the lobbying organization for older Americans.