Mayor Vincent C. Gray speaks during a news conference in this file photo. (J. Scott Applewhite/Associated Press)

Allegations that a D.C. businessman secretly funded a half-million dollar shadow campaign for Hillary Rodham Clinton prompted supporters of Mayor Vincent C. Gray (D) to begin recalculating whether Gray should run for re-election — and whether he could win.

Jeffrey R. Thompson allegedly spent $608,000 to help Clinton in swing states during the 2008 primary. Court records and interviews suggest that the candidate was not aware of the unreported gifts.

It’s a remarkably similar defense to the one Gray has offered over the two-year investigation into his 2010 mayoral campaign. Gray has said he was unaware of Thompson’s $653,000 effort on his behalf, even as several top campaign aides have pleaded guilty for their involvement.

Several Gray supporters contended Thursday that Thompson’s alleged connection to similar activity on the federal level showed a pattern of illegal activity by the donor — not by the candidate who benefited.

“In one respect, this isn’t about Vince Gray anymore, this is about Jeff Thompson,” said Chuck Thies, a political strategist who has backed both Gray and Clinton. “For the people around the mayor, their take is now that Vince is like Hillary; they were both unwitting recipients of Jeff Thompson’s criminal activities. And that might be able to assuage the concerns of people who are inclined to support Gray.”

The widening field of allegations against Thompson, and the potential opening it provides Gray to talk to voters about the two-year-old investigation, comes as the mayor faces a November deadline to decide whether he will run again. Candidates must begin gathering signatures in early November and declare candidacy by Jan. 2.

In an interview Thursday, Gray declined to speculate if the expanding investigation into Thompson put his case in a better light.

“I don’t know, you’ll have to ask the U.S. Attorney that,” Gray said. “I mean, I haven’t changed my position or what I’ve said.”

Asked if his defense may sound more plausible now to voters, Gray said: “Well, there are probably lots of people who already perceived it the way I’ve said it. And it may add to those numbers, I don’t know.”

News that Clinton’s campaign was embroiled in the Thompson investigation also did little to dislodge ardent supporters.

Peter Rosenstein, a Democratic activist and strong supporter of both Gray and Clinton, said that for him, news that Thompson may have funded a shadow campaign for Clinton changed little about his feelings for either.

“I believe Hillary Clinton’s people that they did not know, and at this point, I believe Gray that he did not know,” Rosenstein said. “I am still open to clear and convincing proof otherwise, but this doesn’t change anything.”

According to interviews and court documents filed Wednesday, Thompson, a former city contractor, financed an independent effort to reach urban voters on behalf of Clinton in Texas and at least three other states during the 2008 Democratic primaries.

Thompson allegedly paid Troy White, a New York marketing executive, more than $608,000 to hire “street teams” to distribute posters, stickers and yard signs beginning in February 2008 to help raise Clinton’s profile during her primary battle with then-Sen. Barack Obama, according to the documents and interviews with several people familiar with the investigation.

Prosecutors have been building a case against Thompson, who has been described in court documents as the financier of a secret campaign for Gray’s successful run for mayor, and court records show that he is the subject of a grand jury investigation.

Neither Thompson nor Gray has been charged, and the mayor has denied any wrongdoing. Thompson has not been named in any of the documents, but several people with knowledge of the case have identified him as the businessman in those documents.

Ann E. Marimow contributed to this report.