An oft-quoted Republican political operative pleaded guilty Thursday to illegally directing a political action committee to buy ads to help a congressional candidate whose campaign he managed — a rare enforcement of federal law that bars candidates from spending money in conjunction with their independent allies.

Federal authorities said the plea deal reached with 34-year-old Tyler Harber — who managed the unsuccessful 2012 congressional campaign of Virginia Republican Chris Perkins — sends a warning to those in politics that they need to keep a separation between candidates and outside groups. Prosecutors hailed the case as the “first criminal prosecution in the United States based upon the coordination of campaign contributions between political committees,” and four legal experts said they could think of no other examples, despite an explosion of spending by independent groups on political races in recent years.

“The prudent course here is for candidates and campaign managers to take it as a signal,” said Tony Herman, a former general counsel for the Federal Election Commission who is now in private practice at Covington & Burling. “I do think it should be taken as a message to them to redouble their efforts to make sure they’re not running afoul of the coordination prohibitions in the Federal Election Campaign Act.”

Coordination among candidates and independent organizations has been illegal for decades, but in recent years, the boundary between campaigns and their big-money allies has blurred. That’s in part because of the surge of spending by outside groups since the Supreme Court’s 2010 Citizens United decision, which allowed corporations and unions to spend unlimited amounts independently on political campaigns.

Advocates for tougher enforcement of campaign finance rules say the increased interplay between candidates and their big-money allies has undermined contribution limits. Candidates for federal office can accept individual donations of up to $2,700 per election, while super PACs and advocacy groups can take unlimited money from individuals and corporations.

“It severely undermines the corruption-prevention attributes of the candidate contribution limit,” said Paul Ryan, senior counsel for the Campaign Legal Center, which has advocated for tighter restrictions on coordination.

The Harber case presents a clear example of candidate-group coordination, but some experts say it might not deter some of the legally gray practices that are increasingly commonplace.

Harber, of Alexandria, admitted in court that he helped create a super PAC and arranged for it to buy $325,000 in ads from a particular vendor to help Perkins’s campaign. That vendor paid Harber a $9,100 commission. Harber also admitted asking a big-money donor to give to his super PAC once that donor had given the maximum in contributions to Perkins’s political committee.

The Republican operative, who was often quoted by news outlets, made moves behind the scenes to hide his wrongdoing — using an online alias and different phone number to avoid inquiries and later lying to the FBI about his involvement with Perkins’s campaign and the super PAC. There were elements of individual greed, too, in Harber’s misdeeds. He admitted, for example, that his super PAC — which was donor-funded — paid $138,000 to his mother’s company for work that was never performed, and he used $118,000 of that cash for personal expenses, leaving $20,000 for his mother to spend on real estate.

Harber pleaded guilty to directing coordinated campaign expenditures and making false statements to the FBI. As a part of his deal, prosecutors agreed not to charge his family members embroiled in the case.

In court, prosecutors — as well as Harber and his defense attorneys — did not identify the candidate for whom Harber worked, the super PAC he founded or the vendor from which he bought ads, although they described the scheme with precise figures and dates and specified the congressional race.

FEC records show that a super PAC called the National Republican Victory Fund purchased $325,000 in ads to influence the race of which Perkins was a part. In a brief phone call, Perkins said that Harber was his “consultant” during the campaign but that he needed to research the matter further before talking with a reporter.

“Let’s just say that this is a surprise,” he said.

Herman said that the FEC is hamstrung in its ability to investigate and substantiate alleged coordination and that Harber’s case could “signal an effort by prosecutors to try to fill the gap resulting from what’s perceived as a lack of FEC enforcement.” But he said Harber’s conduct was “so extreme that it’s probably not a recurring kind of a situation.”

Others said the case was more meaningful. Charlie Spies, a campaign finance lawyer who advises GOP candidates and groups, said the plea “puts to bed the argument that the FEC has no teeth and federal election laws are not enforced.”

White-collar criminal defense attorney Jonathan Biran, who handled campaign finance cases as a federal prosecutor, said the plea will “get the attention of the campaign finance community.”

Fred Wertheimer, president of Democracy 21, which seeks to reduce the influence of big money on politics, said his group will be monitoring the 2016 elections and plans to urge the Justice Department to pursue any other possible violations.

“This is an extremely important development and sends a warning signal to all federal candidates in the 2016 elections that they can no longer ignore the nation’s coordination laws with impunity,” he said.

Assistant Attorney General Leslie Caldwell said in a statement that the Justice Department would “aggressively pursue coordination offenses at every appropriate opportunity.”

Although Harber appears to be the first political operative to be criminally convicted of illegal coordination federally, there have been several significant coordination probes at the state level. The most high-profile case has been in Wisconsin, where prosecutors were scrutinizing possible illegal coordination between Gov. Scott Walker’s campaign and outside groups. That case is still being challenged in state courts.

Harber’s misdeeds ultimately were inconsequential. Democratic incumbent Gerald E. Connolly cruised to reelection over Perkins in 2012, despite the late influx of money coordinated with Perkins’s campaign. A spokesman for Connolly declined to comment for this article.

Harber and his attorneys also declined to comment. He is scheduled to be sentenced in June.