Members of the American Postal Workers Union (APWU) will rally today in support of preserving a six-day delivery service.
Plans have been put in place by USPS executives to ditch Saturday deliveries.
Saturday delivery will be cut later this year unless the move is vetoed by Congress.
The APWU said more than 3,000 of its members will gather in Detroit to listen to Union president William Burrus deliver “a spirited denunciation” of the USPS proposals.
The Rev. Jesse Jackson will also speak, according to a union spokesman.
Burrus said cutting Saturday deliveries “would lead to the demise of the world’s most efficient, affordable and trusted postal system.”
The APWU also believes that Congress should relieve the USPS of an onerous provision of the 2006 postal reform law that requires the Postal Service to pay more than $5bn annually to “pre-fund” healthcare benefits for future retirees.
Burrus continued: “While we will not win every battle, we will do everything in our power to protect the interests of APWU members. Let us individually and collectively determine that every day we will sharpen our arguments, we will improve our means of persuasion, and when all else fails, we will appeal.
“Let us everyday continue the great traditions of the American Postal Workers Union.”