Reporter Nikita Stewart explains Jeffrey Thompson's significance in D.C. politics and connections to key figures. (The Washington Post)

Federal investigators are probing Hillary Rodham Clinton’s 2008 presidential campaign as part of an effort to expand their case against a D.C. businessman alleged to have financed below-the-radar political operations, people familiar with the investigation said.

The businessman, Jeffrey E. Thompson, allegedly provided more than $600,000 to fund secret get-out-the-vote efforts to help Clinton in at least four primary states. Prosecutors do not expect to pursue a separate criminal case against Clinton’s campaign, these people said.

In a wide-ranging effort to expand their 18-month-old corruption case against Thompson, federal investigators have reviewed hundreds of e-mails and interviewed more than a dozen Clinton campaign staffers and supporters, according to people familiar with the probe.

Investigators have focused their questioning on the role of Minyon Moore, a senior campaign adviser, and the extent of her role in arranging a “street teams” operation in Texas, Indiana, North Carolina and Pennsylvania that was financed by Thompson and run by marketing executive Troy White, according to those sources.

These people and others familiar with the investigation spoke on the condition of anonymity because the probe is continuing. An attorney for White declined to comment, as has Thompson’s attorney, Brendan Sullivan.

Secret campaigning for Hillary Clinton

Moore’s employer, the political consulting firm Dewey Square Group, issued a statement on her behalf this week, saying that she “was entirely unaware of any inappropriate activities” and is cooperating with authorities.

An attorney for Clinton’s 2008 campaign, Lyn Utrecht, said in a statement late Wednesday that the campaign “has cooperated fully” in the White case and declined further comment.

In a series of interviews over several months, investigators have asked former Clinton campaign officials whether they knew White or were aware of his election-related efforts, which were not publicly disclosed as required by law, some of those familiar with the probe said.

These people said the questioning also focused on e-mails between Moore and campaign staffers about voter turnout strategies in majority-African American and Hispanic urban precincts, as well as about the campaign’s schedule of public events, signs and literature. In many instances, these people said, Moore sought information from staffers who did not know she was also allegedly helping White’s “street teams” operation.

“Minyon, she was influential in the campaign,” said one former campaign official. “If she came in and asked for something, you kind of perked up and did it. But at no point did I have an interaction with her that led me to believe we were doing something wrong.”

Moore, a longtime Clinton operative and confidante, is part of a close-knit circle of advisers who have had preliminary discussions about Clinton running for president again in 2016, when she would be an overwhelming favorite for the Democratic nomination.

Moore was a senior adviser on Clinton’s 2008 campaign with several responsibilities, including outreach to minority voters, managing relationships with surrogates and recruiting superdelegates.

At the time, she was working at Dewey Square Group as state and local director. Clinton’s campaign paid the firm nearly $420,000 for strategic consulting, according to campaign finance records.

‘Street teams’ effort

The pro-Clinton shadow campaigns were first revealed Wednesday in court documents and interviews in connection with White’s guilty plea to a misdemeanor tax charge.

The get-out-the-vote operation allegedly began in Texas, where Clinton’s survival in the drawn-out battle with then-Sen. Barack Obama was on the line. By early February, Clinton had lost 11 consecutive contests to Obama and was narrowly behind him in the count of pledged delegates. Her campaign was running up debt. Next on the calendar were the March 4 Ohio and Texas primaries. Clinton had an edge in Ohio, but Texas loomed especially large.

Clinton won the Texas primary, 51 percent to 47 percent, but in the end Obama got more delegates out of the state by dominating party caucuses held the same day.

No evidence has emerged ­detailing what White’s “street teams” did on Clinton’s behalf or whether they made a difference. Several former Clinton campaign officials said they had no knowledge of the operation and had never heard of White.

According to a narrative detailed in court documents and interviews, White first pitched his firm’s “street teams” services to Guy Cecil, Clinton’s national political director, in January 2008. Cecil declined the offer.

But Moore disagreed, writing in an e-mail to White and Cecil: “I am piping up saying we need your services. [Guy], let’s [find] some money. I will fight for it.”

Moore then introduced White to Thompson, who directed White to put together a plan detailing costs for services to support Clinton in Texas. White pitched a plan to hire “street teams” and made plans to distribute materials prepared by Clinton’s campaign — including lawn signs, pamphlets and stickers — in such cities as Austin, Dallas, El Paso, Houston and San Antonio, according to a statement signed by White and prosecutors as part of his plea agreement.

White got help from a Clinton supporter in San Antonio affiliated with a group identified as “Civic Organization A” in court documents. The group in question was the League of United Latin American Citizens, or LULAC, according to people familiar with the investigation.

The San Antonio-based supporter, whose name was not revealed, reached out to members of LULAC to help recruit paid canvassers in cities such as Brownsville, Corpus Christi, Lubbock and McAllen. After Clinton’s win in Texas, the Clinton supporter affiliated with LULAC continued to help White find workers in other states, the court filing said.

LULAC, a nonprofit social welfare group that says it is the oldest Latino civil rights organization in the United States, does not endorse candidates, but top members of the organization personally supported Clinton.

Luis Vera, LULAC’s general counsel, said that the organization itself had no role in any efforts to aid Clinton. He declined to comment further, citing the ongoing investigation.

Moore stayed in touch with White, providing him with confidential internal campaign documents, including the itinerary and schedule for a “high-profile individual” who would be campaigning in Texas in support of Clinton, according to court documents.

Campaign law questions

The creation of an off-the-books campaign with the cooperation of a senior campaign adviser could violate campaign finance rules if it was determined that the effort constituted an undisclosed, in-kind campaign donation in excess of federal contribution limits. But legal experts said federal investigators would be unlikely to open a separate case because a five-year statute of limitations has expired.

Prosecutors in the U.S. Attorney’s Office in Washington who have probed the Clinton connection remain focused on building a case against Thompson, whom they allege secretly poured $653,000 into an illegal, off-the-books campaign to elect D.C. Mayor Vincent C. Gray (D), people familiar with the case said.

If there is sufficient evidence to prove that Thompson was engaged in an ongoing conspiracy to secretly influence elections, his alleged efforts for Clinton could be added to any criminal charges. Prosecutors could avoid time limitations on pursuing a case if the past acts were part of an ongoing criminal pattern.

In Texas, the Clinton campaign focused most heavily on Hispanics, knowing that Clinton was at a disadvantage against Obama among black voters. Clinton’s team in Texas expected to lose the urban areas of Houston and Dallas by a significant margin and concentrated organizing efforts on heavily Hispanic south Texas, El Paso and San Antonio.

“I didn’t see anything that looked like it was independent or being conducted by a third party,” one organizer said. He added: “If they were pulling this off, they were doing it far below the radar screen.”

Clinton’s Texas primary victory, both sides believe, was the result of a strong showing among Latinos and whites; the deep political roots that Bill and Hillary Clinton had in the Lone Star State, going back to George McGovern’s 1972 presidential campaign; and the strenuous effort that both Clintons made across the state. Obama campaign operatives later said that Bill Clinton was particularly effective at dazzling voters in the state’s small towns and medium-size cities.

In Texas, Hillary Clinton also aired what is remembered as the most devastating television ad of her campaign, challenging Obama’s national security credentials. It featured the image of a ringing White House telephone as a narrator intoned: “It’s 3 a.m., and your children are safe and asleep. Who do you want answering the phone?”

Veterans of both the Clinton and Obama campaigns expressed skepticism that White’s alleged “street teams” made much of a difference.

“We all knew that the African American vote was solidifying into Obama’s corner and it just made zero sense for any of us, with limited dollars, to go into those communities,” one former Clinton staffer said. “It made no sense as a numbers game.”

Dan Balz, Carol D. Leonnig, Karen Tumulty and Alice Crites contributed to this report.