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Since the raid on Mr. Thompson’s home and office, most of the scrutiny of his fundraising activities has focused on Mr. Gray and local D.C. campaigns, as several D.C. Council members received subpoenas seeking details about donations from Mr. Thompson, Harris and others.

But after Harris‘ guilty plea, U.S. Attorney Ronald C. Machen Jr. noted for the first time publicly that the “conspiracy” that has entangled the Gray campaign also funneled tens of thousands of dollars of illegal campaign cash to both local and federal campaigns.

“Since 2001, this scheme has violated federal and local election law by using straw donors, many of them employees, friends and family members of the conspirators, to make high-dollar campaign contributions that would then be reimbursed with personal and corporate money from the conspiracy,” he said.

When faced with the possibility a campaign has accepted dirty money, or legitimate money from a toxic donor, the candidate has three possible courses of action, if he or she chooses to act at all: transfer the money to the Treasury Department, donate it to a tax-exempt charity or return it to the donor.

Campaign finance specialist Michael Toner, former chairman of the Federal Election Commission, said campaigns “can’t claim willful blindness” and keep donations they didn’t know were problematic when they accepted them but then keep the money when evidence surfaces that the funds could be illegal. Mr. Toner said campaigns have a duty to be diligent and return unlawful contributions.

Ms. McGehee said the natural instinct for most campaign operatives is to wait and see how damaging the connection to the toxic donor could be.

“Just like rotten apples on a tree, some of them are going to fall off,” she said. “It’s symptomatic of a sick system.”

Andrea Noble contributed to this report.