Paul Farmer, the leading candidate to head the U.S. Agency for International Development, is no longer in the running for the job, according to several Hill aides and people in the foreign aid community.

Farmer’s withdrawal would send the administration back to the drawing board to fill a critical position that will be a centerpiece of efforts to overhaul foreign aid.

Farmer, whose work improving health care in developing countries earned him a MacArthur Foundation “genius” award in 1993, has been the only name mentioned in foreign aid circles in recent months. A spokesman for Farmer said he was in Rwanda and could not be reached. The White House declined to comment.

“The continued delays in naming a new head for AID are becoming concerning,” said a Foreign Relations Committee aide.

During a town hall-style meeting at the agency last month, Secretary of State Hillary Rodham Clinton called the delay in naming an administrator “frustrating beyond words” and blamed the White House’s protracted, “ridiculous” vetting process for discouraging several candidates. “Some very good people, you know, just didn’t want to be vetted,” she said.

“It’s not clear to me what the administration’s problem is here,” said Richard G. Lugar of Indiana, the top Republican on the Senate Foreign Relations Committee, adding that he was not critical, just “mystified.”

It is uncertain whether Farmer took himself out of the running or whether the vetting uncovered issues that would complicate his confirmation.

The vacancy persists even as plans to overhaul foreign aid have begun to circulate and gain momentum.

Last month, Clinton announced a review, which the USAID administrator would co-chair. Clinton has called for staffing boosts to rebuild USAID and a closer coordination of diplomacy and development work.

Both the House and Senate are considering changes to overseas aid programs. House Foreign Affairs Committee Chairman Howard L. Berman , D-Calif., is circulating a blueprint for a complete rewrite of the 1961 law that governs foreign aid spending. Senate Foreign Relations Chairman John Kerry , D-Mass., expects to mark up a bill after the August recess that would take more modest measures to strengthen USAID as a first step to broader change.