Mayor Vincent Gray's 2010 campaign benefited from a shadow campaign that cost an unnamed donor more than $653,000, federal prosecutors said Tuesday.

Assistant U.S. Attorney Jonathan Haray said that the Gray campaign used unreported "financial assistance" to find a get-out-the-vote operation.

Haray discussed the underground campaign during a plea hearing for Jeanne Clarke Harris, a one-time consultant to the 2010 campaign of Mayor Vincent Gray. Harris pleaded guilty Tuesday to federal and local charges amid a sweeping probe of campaign finance practices in the District.

Less than 24 hours after prosecutors charged Harris with federal conspiracy and fraud counts — as well as a local conspiracy count — the 75-year-old public relations executive admitted guilt before U.S. District Court Judge Colleen Kollar-Kotelly.

"Guilty," Harris told the judge, who is also overseeing two other cases with links to the Gray campaign.

Although the three charges carry up to 8 1/2 years in prison, Harris' plea agreement calls for prosecutors to seek no more than 37 months behind bars for Harris.

Harris admitted to playing a starring role in a plot to funnel money to the campaigns of Gray and other local and federal candidates for political office. In 2010, court filings show, Harris helped the Gray campaign alone to secure $38,000 in contributions.

But those contributions were not ultimately from Harris, two of her companies or the 16 family members, friends and employees whose names were attached to the donations. Instead, an unnamed "co-conspirator" reimbursed Harris and, through her, the others for the donations.

That co-conspirator funded the shadow campaign, Harris admitted, which another unnamed person designed.

"The money was from co-conspirator No. 1, but the plan was developed by another person," Harris said in open court.

After the FBI began probing Gray's 2010 campaign, Harris and her co-conspirator tried to obstruct the investigation by attempting "to take steps to impede federal officials from obtaining information concerning their involvement."

Harris also claimed more than $900,000 in political expenditures as tax deductions in 2010, even though federal law doesn't consider campaign contributions to be deductible.

Although authorities did not identify the co-conspirator, Harris has close ties to Jeffrey Thompson, a prolific contributor to political campaigns in the District and a major city contractor. Federal agents raided her home and office on the same March night that they searched locations with links to Thompson.

Thompson's attorney, Brendan Sullivan, did not respond to a request for comment after prosecutors charged Harris.

Harris is the third person with close ties to the Gray campaign to plead guilty to federal charges. Although Harris' employment links to the campaign were more far limited and short-lived than the others who pleaded guilty — consultant Howard Brooks and assistant treasurer Thomas Gore — the campaign moved to pay one of her companies $20,000 in August 2010.

The check was later canceled, officials with knowledge of Harris' role said, after senior campaign strategists raised questions about whether she had performed the required work.

Harris will have a status hearing in October.