"It is really bad policy to try to shrink to survive. I feel this is un-American," Schakowsky said, the Chicago Sun-Times reported Monday.
The downtown rally Sunday at Chicago's Federal Plaza drew several hundred protesters, the newspaper said.
The Postal Service under Postmaster General Patrick Donahoe has proposed cutting back on mail delivery for Saturdays. A congressional vote last week ensured Saturday mail service through Sept. 30 of this year.
But the issue is sure to resurface, the Sun-Times said.
Letter carrier union spokesman Michael Caref said that the bulk of the problem lay in a a congressional mandate for the Postal Service to pay more than $5 billion to a fund for healthcare benefits for future Postal Service retirees.
"Eighty-five percent of the losses the Post Office has incurred over the past five years," was due to the mandated payments, Caref said.