Holiday stamps are seen on mail at the U.S. Post Office sort center on December 18, 2014 in San Francisco, California. The U.S. Postal Service will process and mail over one billion cards, letters and packages during the 2014 holiday season. (Justin Sullivan/Getty Images)

ANY TIME an official retires after devoting 40 years of honorable public service to a single institution, it’s an occasion for thanks and respect. And it’s an occasion to listen carefully to what that veteran has to say upon his departure. Case in point: the valedictory remarks of Postmaster General Patrick R. Donahoe on Jan. 6, which reflected the lessons of a lifetime as a U.S. Postal Service employee and manager. Mr. Donahoe’s words properly framed the predicament facing this financially distressed but vital agency as a basic test of democratic governance.

The problems facing the postal service and the changes necessary to address them are well understood, as Mr. Donahoe noted in his speech. The Internet has put the agency’s moneymaker, first-class mail, on a path of inevitable decline while it remains burdened by an inefficient regulatory structure, unsustainable health and benefit costs and congressional micromanagement. Among the solutions are five-day mail delivery, shifting retiree health costs to Medicare and greater management freedom to revise infrastructure and develop new product lines.

The fundamental obstacle, Mr. Donahoe explained, is not conceptual or technical but political: the resistance to change by the various interest groups that benefit from things as they are, groups that wield significant clout on Capitol Hill. “As much as we try to have an elevated conversation about the future of the organization,” he said, “we never get beyond the narrow set of interests that are determined to preserve the status quo.”

The mailing industry, he noted, shortsightedly resists greater pricing flexibility for the postal service, even though “the ongoing lack of reform creates more pressure to raise prices — which is what happened this past year.” As for postal unions, Mr. Donahoe said, their “narrow” focus on preserving existing jobs helps short-circuit the postal service’s attempts to operate more efficiently, such as the innovative retail partnership with Staples stores that was curtailed last year in the face of union protests.

So far, what Mr. Donahoe called the “myopia” of postal-related interest groups has prevailed in Congress, preventing the passage of bipartisan reform legislation aimed at restoring the postal service’s sustainability. Without a more farsighted approach, the agency can, at best, expect to stagger along its current path of missed innovations and underinvestment. The new Congress has the power to revive the reform bill this year and thereby decide whether the U.S. Postal Service thrives and modernizes — or whether this foundational institution of the U.S. economy continues to crumble under the weight of special-interest politics.