The criminal probe into Mayor Vincent C. Gray’s 2010 campaign broadened Monday when a public relations consultant and Gray donor was charged in a far-reaching scheme to thwart campaign finance laws.

Jeanne Clarke Harris, 75, is scheduled to plead guilty in U.S. District Court on Tuesday to charges of funneling funds from businesses owned by Jeffrey E. Thompson to her relatives, friends and employees, who then made donations to various federal and local political candidates, according to court documents and people familiar with the probe.

The scheme was intended to evade legal limits on political contributions, and it is among the largest such “straw donation” conspiracies ever alleged in D.C. government.

Harris’s attorney, Frederick D. Cooke Jr., declined to comment ahead of Tuesday’s plea hearing. Thompson has not been charged, and his attorney, Brendan V. Sullivan Jr., did not return a call for comment.

The charges came four months after federal agents raided the homes and offices of Thompson and Harris, seizing dozens of boxes of documents and hundreds of thousands of electronic records. Authorities’ interest in Harris and Thompson, long known as a major campaign financier to Gray (D) and several other current and former D.C. officials, opened a new and expansive front in a federal probe once thought to be limited to Gray’s campaign.

Harris was charged in a criminal information, a legal document that cannot be filed without the consent of the defendant. It generally signals a plea agreement. Harris is scheduled to appear in court on Tuesday for a plea hearing before the same judge who has been handling most of the defendants in the ongoing investigation into Gray’s campaign.

None of the politicians who benefited from the alleged scheme are named in the document charging Harris. But the document refers to “Candidate A” and describes the person as a 2010 mayoral candidate whose campaign has been investigated by federal authorities. Three people with direct knowledge of the investigation identified Candidate A as Gray. Gray declined to comment.

Gray’s 2010 campaign has been the subject of an ongoing federal probe dating to the spring of 2011.

The charging document also identifies a “co-conspirator” with whom Harris had a “close friendship and professional relationship” and together developed the straw donation scheme. The three people with knowledge of the probe said the “co-conspirator” is Thompson — the sole owner of D.C. Chartered Health Plan and the majority owner of accounting firm Thompson, Cobb, Bazilio & Associates, two District-based companies that do hundreds of millions of dollars of business with the city government yearly.

Monday’s charges are the first related to the Gray investigation that go beyond the allegations of Sulaimon Brown. Two Gray campaign aides have pleaded guilty to taking part in a scheme to pay Brown, a fringe mayoral candidate, to attack then-incumbent Adrian M. Fenty (D), who was Gray’s main opponent in the race.

In addition to the campaign finance violations, the charging document alleges that Harris took “steps to impede federal officials” from obtaining information in their investigation of Gray.

The Harris charging document, like those in the previous prosecutions, does not allege Gray participated in or was aware of any criminal activity. But his campaign is alleged to have benefited from the illegal donations.

In 2010, prosecutors charge, Harris assembled donations of $2,000 each — the legal maximum for an individual to donate to city mayoral candidates — from 16 family members, employees and friends. She also gave donations in her own name as well as two of her companies, Belle International and Details International.

Together, the donations totaled $38,000, and prosecutors allege that all of it was reimbursed by Thompson.

The scheme was not isolated to the Gray campaign, prosecutors say. Over the course of more than four years, Harris and Thompson are said to have conspired to evade limits on contributions to the “campaign committees of various candidates for federal office and political action committees.”

In 2008 alone, Harris is said to have personally given $14,650 to federal campaigns and committees, all of it directed and reimbursed by Thompson, the charging papers say. She is also accused of arranging an additional $6,900 in contributions from a family member, a friend and an employee — all of it, again, reimbursed by Thompson.

None of the federal recipients are named, but federal campaign records indicate Harris gave to seven candidates in 2008, including Del. Eleanor Holmes Norton (D), Rep. Heath Shuler (D-N.C.), former Rep. Albert R. Wynn (D-Md.) and the presidential campaigns of Hillary Rodham Clinton and Barack Obama. Harris also gave a $5,000 contribution to Emily’s List, a Democratic political action committee. The papers do not imply that any of the recipients knew of the scheme.

Details in the charging documents point to a conspiracy that went beyond straw donations.

Prosecutors allege that Harris falsified a 2010 tax return for Belle International by improperly deducting $908,217 in business expenses that were actually used for “political expenditures.”

The charging document does not detail the purposes of those expenditures. But federal authorities have been investigating what campaign workers have called a “shadow campaign” waged on Gray’s behalf that used money that was not properly reported under campaign finance laws. Two people familiar with that aspect of the probe say the spending could have exceeded $600,000.

Workers and volunteers have previously told The Washington Post that Harris helped direct the off-the-books effort with Vernon Hawkins, a longtime friend of Gray’s.

A person familiar with the probe said Harris entered final negotiations with prosecutors Thursday. She signed a plea agreement Monday, the person said.

Harris is described by friends, employees and relatives as hard to miss — stout, chain-smoking and sometimes sporting purple hair. On Jan. 8, Gray issued a ceremonial proclamation commemorating Harris’s 75th birthday, a formal request by Linda Mercado Greene, Gray’s girlfriend.

Harris has a long history in city politics, dating back to a stint as the campaign press secretary for Mayor Walter E. Washington’s 1978 reelection campaign. Through the 1980s, Harris’s public-relations firm won contracts with District agencies, and she attracted attention when she hired Effi Barry, the wife of then-mayor Marion Barry, in 1984.

Effi Barry quit three years later, but Harris became embroiled in another scandal involving former council member H.R. Crawford (D-Ward 7). She pleaded guilty in 1988 to concealing a scheme to pay a Crawford friend $20,000 through a city contract Harris held.

After her 1989 sentencing — she received two years’ probation and a small fine — a Post reporter asked Harris whether she had learned a lesson from her prosecution. “Yes,” she said. “Be careful.”

Del Quentin Wilber contributed to this report.