CREW had first spotted the payment buried in the campaign's FEC filings, and publicized it earlier this week.
Asked about the donation Monday by TPMmuckraker, Marc Elias, a lawyer for the campaign, declined to explain it, citing the need to avoid compromising the privacy of the original donor from whom the campaign received it. Elias described the donation as "routine."
But several experts took issue with that. One veteran Washington campaign-finance lawyer called Elias's claim "bullshit."
"Campaigns don't routinely cough up money to the Treasury," he told TPMmuckraker. "FEC regulations require campaigns to return questionable contributions to the contributor -- not send them to the Treasury Department."
The campaign-finance lawyer continued:
The only time I've seen an amount this large sent to the Treasury was when the FEC found that the contributions were the result of an illegal corporate contribution scheme and the FEC ordered the campaign to disgorge the contributions to the U.S Treasury.
Craig Holman, a campaign-finance specialist at Public Citizen, agreed, telling TPMmuckraker that payments of that size by Senate campaigns to the US Treasury "can't be called routine," though he said they have been known to occur.
Reached again by TPMmuckraker, Elias stood by his original characterization of the donation.