Mayor Vincent C. Gray and D.C . C ouncil member Marion Barry together in July. Barry has lent his support to Gray’s reelection campaign. (Nikki Kahn/The Washington Post)

A widely respected supporter of Mayor Vincent C. Gray expressed new concerns about his possible involvement in campaign wrongdoing Tuesday, a day before he was to receive the endorsement of iconic former four-term mayor Marion Barry.

Alice M. Rivlin, a former Clinton administration budget director who is now a Brookings Institution researcher, was a high­profile supporter of Gray’s run against then-Mayor Adrian M. Fenty (D). She hosted a fundraiser in her Forest Hills home, and she lent her expertise to Gray’s transition committee.

Reached on Tuesday, Rivlin indicated that her support is wavering in light of prosecutors’ recent allegations that Gray (D) knew of an illegal, off-the-books campaign waged on his behalf in 2010.

“I don’t know what to believe,” she said. “I’ve been a strong supporter of the mayor. I think he’s run the city very well, and I’m in the quandary that everyone else is in. . . . I’m not sure I need to say more than that.”

Rivlin declined to say whether she plans to vote for Gray in the Democratic primary. She has served as an adviser and sounding board during Gray’s administration and played a leading role in the search for the city’s new chief financial officer.

Federal authorities laid out the allegations after negotiating the guilty plea of businessman Jeffrey E. Thompson, who admitted orchestrating a “shadow” campaign for Gray four years ago. Thompson implicated the mayor in the scheme. Gray denied the allegations and pitted his credibility against that of Thompson, who was once the city’s largest contractor.

“Who do you believe?” he asked in his State of the District address, a day after Thompson’s plea. “A greedy man attempting to save himself? Or me, a public servant who has dedicated my entire career and my entire life to giving back to our communities in the District of Columbia?”

Rivlin’s statement, a discordant note among Gray’s generally loyal longtime supporters, came two weeks before primary votes are counted April 1. Early voting continued for a second day Tuesday, with more than 500 people casting ballots at a downtown polling place, according to the D.C. Board of Elections Web site. That was a big jump from the 158 who braved Monday’s wintry weather to cast ballots.

Chuck Thies, Gray’s campaign manager, declined to address Rivlin’s doubts. But he said Gray’s support is “strong and growing,” citing a number of labor endorsements.

“We continue to receive an outpouring of support from across the District, and given the coverage of this campaign, it doesn’t surprise us that some people may be concerned,” Thies said. “Ultimately, voters know the truth and will look at the mayor’s record and his accomplishments on fact as opposed to innuendo.”

Gray is expected to publicly tout Barry’s support Wednesday, days after the D.C. Council member and “mayor for life” gave his first interviews in support of the embattled Gray and stumped for him on Barry’s home turf, Ward 8.

Although the endorsement comes as little surprise, Barry (D) had kept his mayoral preferences to himself until relatively recently, and Gray’s opponents — particularly council colleague Jack Evans (D-Ward 2) — had sought to woo him to their camps.

Four years ago, Barry vigorously campaigned for Gray, but a formal endorsement event was not staged as Gray sought to avoid being tied too closely to a previous era of District politics. Since then, Gray and Barry have tangled occasionally, but they have maintained a cordial relationship. In the crowded primary, Gray is depending on racking up big margins among Barry loyalists.

Gray was present in Barry territory Tuesday to cut the ribbon on a community health center in the Bellevue neighborhood of Southwest Washington.

The Barry news was first tweeted Tuesday by WUSA (Channel 9) reporter Bruce Johnson, and the Gray campaign shortly afterward issued an embargoed advisory listing details of the event.

A Post reporter inadvertently broke the embargo by tweeting about the advisory, but The Post independently confirmed the impending Barry endorsement with a person familiar with the mayor’s plans. LaToya Foster, a spokeswoman for Barry, said he intends to announce his endorsement Wednesday afternoon at Matthews Memorial Baptist Church, near the Anacostia Metro station. The pastor, C. Matthew Hudson, is an ally of Barry’s and Gray’s.

Hudson delivered the invocation at Gray’s State of the District address last week, and he chairs the board of United Medical Center, the city-owned hospital in Ward 8.

Barry, 78, recently left an inpatient physical rehabilitation center, wrapping up two lengthy stays in medical facilities. He continues to undergo physical therapy and has yet to fully return to his duties as council member.