Leiken paid his mother’s firm, P&G Marketing and Research, the money to interview 200 Springfield residents about a proposed new gas tax in February and has been accused of campaign finance violations because the agency was not properly registered.

At a news conference covered by local press, Leikin said: “Rather than try to justify or prove my innocence, I will say this: I didn’t document [an expenditure] transaction correctly. But let he or she who has never made a mistake cast the first stone. That’s the statement I wanted to make.”

Leikin’s mother’s firm said it had made 268 calls to interview 200 people, but other pollsters cited in reports in the local Register-Guard cast doubt on the response rate that these figures imply.

A memo sent to state officials by P&G Marketing and Research as part of the investigation said that the survey was a “snapshot analysis” rather than a scientific study.

Leikin, who is currently running for Congress, has repaid the cash and apologised to constituents and family for “creating a shadow of doubt”.

The investigation is ongoing.