A former D.C. Council candidate pleaded guilty Tuesday to conspiring with businessman Jeffrey E. Thompson to conceal tens of thousands of dollars in political spending in violation of city campaign finance laws.

Kelvin Robinson is the second former political candidate to acknowledge secret donations from Thompson, who has admitted financing an illegal “shadow” campaign in support of Vincent C. Gray’s successful 2010 mayoral bid.

The conspiracy charge filed against Robinson last month and his plea agreement announced in D.C. Superior Court on Tuesday signal that the long-running federal corruption investigation has entered a new phase, focused on candidates who had knowledge of — and benefited from — Thompson’s secret contributions.

Thompson, once a major city contractor, has been cooperating with federal investigators after admitting in March that he illegally pumped more than $2 million into the campaigns of more than two dozen federal and local candidates during a six-year period. His allegation that Gray helped orchestrate the off-the-books “shadow” spending in 2010 came just weeks before the mayor was defeated in the April Democratic primary.

The mayor has not been charged with a crime and has accused Thompson of lying about Gray’s alleged involvement in the campaign finance scheme.

Kelvin J. Robinson (Courtesty of Kelvin J. Robinson)

But U.S. Attorney Ronald C. Machen Jr. appeared Tuesday to be making good on his warning at a news conference after Thompson’s plea hearing. At the time, Machen said that his office was “not going away” and that anyone involved with Thompson’s covert spending should “come forward and own up.”

In the past two years, seven other people connected to Thompson or affiliated with Gray’s 2010 campaign have pleaded guilty in federal court. Last week, former D.C. Council member Michael A. Brown was sentenced to more than three years in prison in a bribery case in which he acknowledged taking illegal contributions from Thompson in several campaigns.

In court Tuesday, Robinson, once chief of staff to then-Mayor Anthony A. Williams, admitted that he had failed to properly report more than $33,000 in donations as required by D.C. law. Prosecutors said Robinson received more than $7,500 from Thompson in his aborted bid for an at-large seat in 2010. Thompson also secretly contributed about $26,000 to help Robinson in an unsuccessful campaign for the Ward 6 council seat, according to court papers.

Campaign finance laws in the District limit contributions from individuals or businesses to $1,000 in at-large races and $500 in contests for ward seats.

Robinson, 53, was first contacted by law enforcement officials months before Thompson pleaded guilty, and he quickly acknowledged his conduct, defense attorney Vandy L. Jamison Jr. said Tuesday.

“He is willing to accept responsibility. It was the appropriate thing to do,” Jamison said after the hearing.

The money Thompson spent on Robinson paid for a long list of get-out-the-vote supplies, including yard signs, stickers, banners and T-shirts. Prosecutors said Thompson, through an associate, also paid to rent a campaign office for Robinson and to send direct mail to voters.

Robinson’s campaign finance reports “misrepresented and concealed the excessive and unreported in-kind contributions provided directly and indirectly by Thompson,” Assistant U.S. Attorney Ellen Chubin Epstein told Judge Anita Josey-Herring on Tuesday.

In the fine print of Robinson’s plea deal, there were echoes of how prosecutors say Thompson operated on behalf of other campaigns, including Gray’s 2010 bid. The financial boost for Robinson was funneled through companies owned by Thompson’s associate, Jeanne Clarke Harris, who pleaded guilty in 2012 to making unreported payments to help finance Gray’s campaign.

Prosecutors also said that in May 2010, Robinson presented Thompson with a budget of about $90,000 to cover expenses for his Ward 6 bid. In Thompson’s case, prosecutors said in court papers that Gray handed Thompson a budget with his get-out-the-vote expenses and “expressed gratitude” for the assistance.

Thompson also said in court that many of the candidates he supported knew about his unreported “shadow” campaigns. But Machen has made a point of noting that not all of the candidates Thompson secretly backed were aware of the illegal spending.

In the days after Thompson’s plea deal, Machen was criticized by Gray supporters for public remarks that some viewed as a direct message to the mayor, and because prosecutors identified Gray in court even though he had not been charged.

A court order unsealed late Monday shows that U.S. District Judge Colleen Kollar-Kotelly ordered the government to publicly name the mayor during Thompson’s court hearing, writing that Gray and his campaign were integral to the misconduct that Thompson acknowledged in his plea agreement with prosecutors.

In Robinson’s case, the plea deal calls for jail time ranging from one to 12 months. A sentencing hearing is set for Aug. 13.

Mila Kofman, executive director of the D.C. Health Benefit Exchange, where Robinson works, said the government office is reviewing Robinson’s employment status in light of his legal troubles. In a statement when the charges were filed, Kofman also praised Robinson’s work in business development as being “of the highest caliber.”

Jamison, Robinson’s attorney, said his client’s decision to accept responsibility should “not disqualify him from his job.”

Mike DeBonis contributed to this report.

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