Requiring future retirees to enroll in Medicare could save the postal retirement and health programs about $5.6 billion through 2031.

"It's not fair to say that we have to let someone gouge us in order for them to be in business," Becerra said.

Opponents argue that the rule for implementing the law favors insurers and doesn't follow the spirit of the legislation.

The massive spending bill is likely to at least delay the so-called Medicare rebate rule released at the end of the Trump administration.

A large portion of the latest $3.5 billion budget proposal would be dedicated to health care, targeting major sections of the system.