PAC Led by Postal Official Pours $20 Million Into Rescuing Tillis

WASHINGTON — The Republican “super PAC” American Crossroads plans to pump about $7 million over the next month into a bid to secure Senator Thom Tillis’s North Carolina re-election and save the party’s Senate majority, raising fresh questions about partisanship at the United States Postal Service, where one of the political group’s leaders holds a powerful post.

The infusion — which leaders of the group said would bring its total investment in the critical race to more than $20 million — will pay to blanket the airwaves in the state with advertisements attacking Cal Cunningham, the Democrat challenging Mr. Tillis in a contest widely seen as a linchpin for control of the Senate. The deep involvement of American Crossroads, which is led in part by Robert M. Duncan, whom President Trump installed at the helm of the Postal Service’s board of governors, is the latest example of a potential conflict of interest at the highest levels of the agency charged with carrying out vote-by-mail operations for the November elections.

Mr. Duncan founded the political group and sits on its board of directors, but its spokesman said he is not involved in day-to-day operations. Still, his position has underscored how a tight-knit circle of Republican donors has risen to power at the Postal Service, and fueled concern among Democrats and government ethics specialists that their partisan preferences could compromise its independence.

“There really is a reason to feel like this almost sacred institution in American life is being politicized, and these continued actions by Robert Duncan don’t do anything to dispel that,” said Noah Bookbinder, the executive director of Citizens for Responsibility and Ethics in Washington, a liberal government watchdog group.