A day after prosecutors accused Mayor Vincent C. Gray of participating in an illegal, off-the-books campaign in 2010, a broad collection of loyal supporters said Tuesday that they will stand by him as the April 1 Democratic primary approaches.

About a dozen supporters — contributors, political operatives, labor leaders and grass-roots activists — said consistently in interviews that they trust the mayor’s denials and question the allegations of businessman Jeffrey E. Thompson.

“I stand with him,” said Carrie Thornhill, a high school classmate and a longtime friend and neighbor of the mayor’s. “He’s fully trustworthy. I do not believe that he should resign or pull out of the race. Absolutely not. He’s been a good mayor, and he’s a good human being.”

Thompson, who admitted in federal court that he financed an illegal campaign scheme, told prosecutors that Gray knew of it. The mayor’s supporters lashed out at Thompson and described him as the beneficiary of a sweetheart deal that lets him off easy for serious acts of corruption involving many local and federal campaigns.

The avowals of support indicate the depth of goodwill — at least among longtime friends and some of the city’s power brokers — that Gray can draw on in the final weeks of the primary campaign. But it remains unclear whether the calls for loyalty will resonate with the broader array of city voters Gray needs on his side to win.

In a week that began with him ahead in the polls and with more campaign cash than any of his opponents, the mayor now faces tougher questions than ever before about his trustworthiness.

The Rev. Graylan Hagler, who backed Gray four years ago but is undecided this time, said, “A lot of people . . . are trying to make sense of this.” He added, “The question is: Can Gray keep together his base?”

Gray began shoring up support among his most-loyal labor allies just hours after the new allegations were aired in a downtown courtroom Monday.

Geo T. Johnson, head of a major city employee union that has endorsed Gray for reelection, said he told the mayor that Thompson’s allegations were “nothing more than news-grabbing attention.”

As for Gray, Johnson said: “He said, he’s telling people the same thing he was telling people six, eight months ago. He ain’t changing his story. I believe him.”

In court, Thompson said Gray personally presented him with a one-page budget for more than $400,000 in “shadow” spending ahead of the 2010 Democratic mayoral primary, with the understanding that the spending would be kept from the public to protect Thompson’s business interests.

Gray, in interviews Monday, decried Thompson’s allegations as “lies.” That was enough for several of his closest allies, who said Thompson was less believable.Thornhill, who introduced him at his January campaign kickoff, said she had no reason to doubt Gray’s version of events.

Several Gray supporters questioned the terms of a plea deal that could result in Thompson’s serving six months or less in prison. The maximum sentence for the two felony conspiracy charges is five years each. The lenient treatment, several Gray supporters said, is evidence that Thompson lied about the mayor to avoid an extended jail term.

“When you’ve spent your entire life buying power and influence, and you can’t buy your way out of this one, that’s what Jeffrey Thompson did,” said MaryAnn Miller, a Gray campaign supporter. “The mayor has spent his entire life working for the people of this city and doing the right thing, and he is being massacred by people like a Jeff Thompson.”

Miller, who volunteered on the 2010 campaign, said Gray’s version of events — that he was betrayed by campaign aides acting without his knowledge or authorization — comported with her experiences in 2010.

“I knew nothing about these kinds of activities,” she said. “It’s unfortunate that people misled him, that people betrayed him. . . . He never would have allowed this.”

Aviva Kempner, a filmmaker, hosted a “listening session” for Gray in the fall while he was deciding whether to seek a second term. She said she has no plans to remove the Gray campaign sign from her front yard.

She, too, had knives out for Thompson. “Jeff Thompson came to this city and treated it like a banana republic,” Kempner said. “I think it is shocking he is only going to get six months for trying to corrupt the political system. . . . I’ll leave that at that.”

Gray said Monday that “you wonder about the timing of something like this,” and several supporters, following his lead, have asked why Thompson came clean so close to the April 1 primary election. His plea was entered one week before early voting begins.

D.C. Council member Yvette M. Alexander (D), who filled Gray’s Ward 7 council seat when he rose to chairman in 2007, called the timing “odd.”

“It just makes you wonder," she said. “I can’t explain it. I don’t know their process.”

When prosecutors announced Thompson’s plea deal Monday morning, Gray had just finished a breakfast fundraiser at the downtown law offices of a fraternity brother, Kenneth Trombly.

On Tuesday, Trombly said he was still shaking his head over the timing. “If you really dig down on this . . . the prosecutor says they have this strong case, but the only evidence is the testimony of Mr. Thompson, whose credibility is pretty questionable,” he said.

“If that’s all they’ve got, why did they wait so long?” Trombly added. “It almost seems inappropriate to bring charges that could have been brought at a less sensitive time.”

What remains unclear is whether voters who might not have a personal connection to Gray but have been inclined to support him will be willing to look past the new allegations, which could put him at risk of criminal charges.

U.S. Attorney Ronald C. Machen Jr. said Monday that Thompson’s allegations represent the “tip of the iceberg” in a continuing corruption investigation, with campaign finance misdeeds representing only part of the probe.

Rahim Jenkins, Gray’s deputy campaign manager for heavily African American wards east of the Anacostia River, said that when he took the temperature of supporters there Tuesday, little had changed.

“Fortunately, most of the people I know believe in the fact that you are innocent until proven guilty,” said Jenkins, who is on leave from a job in the Youth Rehabilitation Services Department and has a long history advocating for returning prisoners. “It doesn’t cause any of the people I’ve been talking to to be overly concerned.”

Two polls published since the beginning of the year — one by The Washington Post, another by a coalition of local media outlets — showed Gray with the support of one-quarter to one-third of likely primary voters. That is weak for an incumbent but significantly ahead of any challenger in the crowded field.

J.R. Meyers, a political consultant who is backing Gray, said that he believes the mayor’s support is “baked in” and that recent fundraising will help turn out loyal voters. “I don’t think it changes anything,” Meyers said of the Thompson allegations.

Campaign finance reports filed Monday showed Gray with the largest bankroll going into the final weeks, slightly ahead of D.C. Council member Muriel Bowser (D-Ward 4). Bowser also led other challengers in the recent poll by WRC (Channel 4), WAMU (88.5 FM) and the Washington Informer newspaper.

Meyers said he doubted that Gray supporters would bolt for Bowser, who is closely associated with former mayor Adrian M. Fenty, whom Gray beat in the 2010 Democratic primary. “The mayor’s faithful wanted to beat Fenty,” he said. “They aren’t turning back on that philosophy. Fundamentally, Fenty is still in the race.”

Tom Lindenfeld, a political consultant for Bowser, questioned that reading of the race. He said Gray needs to grow his support beyond the 25 to 30 percent he currently enjoys to win. “I don’t think he gained any new votes yesterday,” Lindenfeld said.

Gray’s union backers, whom his campaign is relying on for manpower, say they will continue to work for him. John Boardman, secretary-treasurer of Unite Here Local 25, which represents thousands of city hotel workers, said, “We’re still with the mayor.”

“I think this is relatively new information,” Boardman said of Thompson’s claims. “I don’t think that changes what the mayor is about and what he wants to do.”

Johnson, the leader of District Council 20 of the American Federation of State, County and Municipal Employees, said he plans to put as many as 50 union members on the street in wards 7 and 8 this weekend to canvass for Gray. “We’re riding this horse all the way to the finish line,” he said.

One rank-and-file Gray supporter said she is standing firm — for now. Ronnie Kweller, a public relations specialist, voted for Gray last time and said she believes he is a good man who deserves the benefit of the doubt.

“I’m not ready to condemn him until I see some evidence what Jeffrey Thompson said in court was actually true,” she said. “Where is that one-page budget that the mayor supposedly gave to him? Where’s a credible witness? Where is the paper trail?”

But Kweller said she had planned to vote during the early-balloting period that starts Monday. Now, she said, “I’m waiting. . . . I’m not sure if I’ll vote early, but at the moment, I’m still inclined to vote for him.”