A postal worker whose allegations of ballot tampering are the basis of Republican calls for investigations has reportedly recanted his story.

Democrats on the House oversight committee said that Richard Hopkins – the worker who claimed in a signed affidavit that a supervisor at the US Postal Service (USPS) in Erie, Pennsylvania, instructed staff to tamper with ballots by backdating ones that arrived late – had recanted the allegations in an interview with investigators for the USPS inspector general.

Investigators told the committee that Hopkins “did not explain why he signed a false affidavit”, the committee wrote in a statement.

Hopkins admitted to fabricating his claims, the Washington Post reported on Tuesday, citing three officials. After he submitted the affidavit, the South Carolina Republican senator Lindsay Graham, who heads the Senate judiciary committee, called for a federal investigation.