Thomas Gore, 56, assistant treasurer for Vincent Gray’s 2010 mayoral campaign, leaves the U.S. District Courthouse on Tuesday. (Katherine Frey/THE WASHINGTON POST)

A key finance official on Mayor Vincent C. Gray’s 2010 campaign admitted in federal court Tuesday that he helped to secretly finance the campaign of a minor candidate to bolster his boss’s prospects and then destroyed a ledger detailing the payoffs.

Thomas W. Gore pleaded guilty to three D.C. election law misdemeanors and a federal charge of obstruction of justice, and agreed to cooperate with federal prosecutors in theirwide-ranging investigation of Gray’s mayoral campaign.

Gore, a longtime confidant of Gray, is the first person to admit wrongdoing in the ongoing federal investigation, and he faces 12 to 18 months in prison under sentencing guidelines. No sentencing date has been set, and depending on Gore’s cooperation, prosecutors might seek to further reduce his sentence.

Wearing a dark suit and tie, the 56-year-old Gore was subdued during a lengthy hearing, answering, “Yes, your honor” and “I understand” to questions posed byU.S. District Judge Colleen Kollar-Kotelly.

“How do you plead?” the judge asked at the end of the 90-minute hearing.

“Guilty,” Gore replied.

Although federal prosecutors did not name Gray (D) or his campaign in court records, Kollar-Kotelly pointedly asked Gore to identify what was described in the plea documents as “Candidate A” and “Campaign A.” Hesitating, Gore finally said, “the campaign for Vincent Gray.”

Surrounded by friends and family as he left the District’s federal court, Gore declined to comment. His attorney, Frederick D. Cooke Jr., said Gore “pleaded guilty because he wanted to accept responsibility for his role in this endeavor and he thought it was the right thing to do. He wanted to acknowledge his missteps.” Cooke added that he expected others to be charged in the investigation.

A friend of the mayor’s for two decades, Gore was treasurer of Gray’s 2004 D.C. Council run and his successful bid two years later to become the body’s chairman. As assistant treasurer of Gray’s mayoral campaign, he handled the day-to-day finances, Justice Department prosecutors said.

To enhance Gray’s chances during the Democratic primary, Gore and others on the campaign began scheming as early as June 2010 to keep minor candidate Sulaimon Brown in the race so he could continue to assail incumbent mayor Adrian M. Fenty (D) on the campaign trail. Their solution, prosecutors said, was to funnel“excessive or unattributed” campaign cash to Brown through money orders.

In June and July of that year, Gore used excess campaign funds to purchase $660 in money orders from stores near the campaign’s headquarters. He then gave the money orders to a member of the campaign who is identified only in court papers as “Person A.” However, people familiar with the investigation but not allowed to speak publicly said that person was Howard L. Brooks, a consultant who was paid $44,000 by the Gray campaign.Brooks, who was also a longtime golfing buddy of Gore, has not been charged.

To disguise where the money originated, Person A signed the names of family members on four of the money orders and had a relative sign the name of a friend on a fifth one. Person A then delivered the payments to Brown, who deposited them in his campaign bank account, prosecutors said.

Gray defeated Fenty in the primary and cruised to a general election victory in November. The Washington Post reported in March 2011 that Brown claimed to have received cash and money orders from the Gray campaign to stay in the race and disparage Fenty. In exchange, Brown claimed, he was given a $110,000 job in the Gray administration. Gray has denied Brown’s allegations, which sparked the federal probe that has since become an inquiry into alleged campaign-finance abuses.

After reading The Post story, which prosecutors referred to in court papers as “media reports,” Gore shredded a spiral-bound ledger that recorded the illicit payoffs, court records show.

He also admitted that he lied to federal investigators months later when he told them he had never kept a ledger and had not told anyone about the notebook. In fact, prosecutors said, he discussed the ledger and admitted destroying it in a conversation with Brooks.

Gore told the judge Tuesday that his comments about shredding the ledger were captured on “a wire.” At the time, Brooks was cooperating with federal authorities and wore a recording device to that meeting with Gore, according to people familiar with the probe.

Brooks could not be reached Tuesday, and his attorney, Glenn F. Ivey, declined to comment.

Staff writer Nikita Stewart contributed to this report.