Updated

The unions are pushing back hard against Postal Service plans to end Saturday mail delivery, rallying across the country Sunday and launching TV ads to bolster their case that America needs its mail six days a week.

The opposition is mounting after Postmaster General Patrick Donahoe announced last month that he intended to wind down Saturday mail. He called the move "absolutely necessary" as part of a broader effort to stabilize the service's troubled finances.

Under the proposal, the Postal Service will continue to deliver packages six days a week. The plan, which is aimed at saving about $2 billion, would start to take effect in August.

But Congress -- which oversees the otherwise independent agency -- presumably must approve the plan.

An ad from the National Association of Letter Carriers tries to put pressure on congressional lawmakers.

"We the people ... want Congress to know that we depend a great deal on all we get from the U.S. Postal Service. Our mail matters. We need Saturday delivery," the ad says.

The union also helped organize a nationwide set of rallies Sunday to protest the decision.

The move to halt weekend mail is nothing new. Over the past several years, the Postal Service has advocated shifting to a five-day delivery schedule for mail and packages -- and it repeatedly, but unsuccessfully, appealed to Congress to approve the move.

Under the new plan, mail would still be delivered to post office boxes on Saturdays. Post offices now open on Saturdays would remain open on Saturdays.

Though an independent agency, the service gets no tax dollars for its day-to-day operations but is subject to congressional control.

The Associated Press contributed to this report.