Politicians in states around the country have moved in recent years to rein in the pensions of government employees, which in many cases had become more generous and less risky than those of their private sector counterparts.

Now that movement may be breaching yet another firewall: the pensions of federal employees.

On Thursday, the board of the Tennessee Valley Authority Retirement System, the pension program for roughly 11,000 workers and 24,000 retirees at the venerable New Deal-era agency, approved a tentative plan to lower the system’s funding shortfall by reducing benefits.The plan will be implemented later this year if the T.V.A.’s management and board go along with it.

The immediate impetus for voting on the changes was a proposal put forth in December by William D. Johnson, the agency’s chief executive, who argues that demographic trends and the retirement system’s historical generosity make it unsustainable. Mr. Johnson’s proposal, a modified version of which the board embraced, called for shifting many employees from a pension that guarantees a fixed level of benefits, a feature of most federal employees’ retirement package, to a 401(k) plan. It would have also lowered the cap on cost-of-living increases even for current retirees, effectively cutting a benefit they had already earned.

But what has elevated an increasingly common debate about pensions into a larger controversy about inequality is Mr. Johnson’s decision to exempt from the cutbacks the benefits that he and other executives receive through a supplemental retirement plan. (Their benefits in the general T.V.A. pension plan would be subject to the same cuts as other workers.)