Gretchen Whitmer, who, on Tuesday, became the Democratic nominee for governor of Michigan, is one of the many liberal politicians who on some issues are moving toward the left. She campaigned on a fifteen-dollar-an-hour minimum wage and universal pre-kindergarten. But Whitmer is also a member of Michigan’s political establishment: she was a member of the Michigan Senate from 2006 to 2014, and for four years its minority leader; she is the favored candidate of the state’s still-powerful unions.

She is also the daughter of Richard Whitmer, who served in a Republican governor’s cabinet and, from 1988 to 2006, was the president and C.E.O. of Blue Cross Blue Shield of Michigan—a biographical detail that came to be pivotal in the campaign.

Whitmer had two primary opponents, both of whom had grown up much farther from political power and seemed more interested in thoroughly upending it. Shri Thanedar, a sixty-three-year-old Indian immigrant and chemical-testing entrepreneur, committed to spending ten million dollars of his own money to position himself as an unabashed progressive. Abdul El-Sayed, a thirty-three-year-old former executive director of the Detroit Health Department, said that he was inspired to run for office after seeing the state’s mismanagement of the Flint water crisis, and was endorsed by Bernie Sanders. Thanedar had the appearance of a vanity candidate—there was even a report, which he hotly denied, that in early conversations with political consultants he had considered running as a Republican—but El-Sayed was a more idealistic and ambitious figure: the son of Egyptian immigrants, he went to Oxford as a Rhodes scholar, where he received a doctorate in public health. El-Sayed, who supported a single-payer health-care program, campaigned as if convinced that Whitmer’s Obama-era liberalism would no longer seem progressive enough for Michigan, a state whose Democratic-primary voters had backed Sanders for President. As his campaign gathered momentum, it also operated as a probe into liberal politics—into what its weak points are in this new area, and how they might be defended.

The conflict between Whitmer and El-Sayed began to emerge early, during the primary race’s long, slow spring. In late February, the Detroit News reported that four Blue Cross Blue Shield executives had planned a Whitmer fund-raiser for March 7th, and that the company’s political-action committee had given her campaign “one-time permission” to invite its eight thousand employees. The event raised a hundred and forty-four thousand dollars for Whitmer. A tax-exempt organization called Build a Better Michigan, a so-called 527 group that was able to accept donations of unlimited size, began airing ads backing Whitmer in June, spending $1.8 million. Under federal rules, the group was not required to report its donors to the Internal Revenue Service until July 15th, and the agency likely would not make them public until after the primary.

Questions about Blue Cross’s support of Whitmer, and how much more the company might have donated through Build a Better Michigan, were already circulating when, on June 6th, El-Sayed sharpened his critique. At a press conference in Detroit, he proposed a single-payer health-care program for Michigan—and took repeated aim at Blue Cross, which controls seventy per cent of the state’s health-care market. “Their C.E.O. made thirteen million dollars last year, while six hundred thousand Michiganders didn’t have access to health care,” he said. El-Sayed did not mention Whitmer, who did not endorse a single-payer system, by name, but he said, “Now they’ll tell you it can’t be done. Well, I would say if you’re taking corporate bribe money from the big corporations who don’t want it to be done . . . Yes, it can’t be done.” (Whitmer strongly objected to El-Sayed’s claims of corruption throughout the campaign. Blue Cross officials repeatedly declined to comment on donations to Whitmer.)

Purity in politics can be hard to prove. It has grown even harder in the past decade, as new state and federal campaign-finance laws have helped to conceal many donors, and Michigan presents an especially dramatic case. As Craig Mauger, the executive director of the Michigan Campaign Finance Network, a nonprofit organization that tracks money in campaigns, told me, the state is “pretty notorious for the amount of dark money that pours into our races.” In 2015, the Center for Public Integrity ranked it fiftieth out of fifty states for transparency and accountability in state government. Many of the candidates running in Sanders’s wake in Democratic primaries across the country have distinguished themselves by their refusal to take donations from corporate political-action committees, and their attacks on opponents who do. But, because so many campaign donors are only partly visible, the charge of corporate influence, once raised, can be hard to prove or disprove—it just hangs there, like a stench. In a July debate, El-Sayed made his attack on Whitmer’s Blue Cross ties sharper still, saying, of the March fund-raiser, “We don’t know what was said. We don’t know what kind of deals were cut. That’s the kind of corporate corruption we cannot sustain in our health system.” Whitmer, by way of rebuttal, noted that El-Sayed’s campaign had taken a hundred and seventy thousand dollars in individual donations from corporate executives. “You can’t be half-pregnant on this one,” she said.

One day this summer, in the midst of this escalation, while he was scrolling through political sites on his phone, a Democratic operative in Lansing named Bob McCann came across a fifteen-second video advertisement attacking Whitmer. McCann spent several years working as Whitmer’s chief of staff when she was the minority leader of the state senate, and he considers her a close friend. When he encountered the ad again, he unmuted it. “Who is Gretchen Whitmer?” a female narrator asked, over a screen split between an image of Whitmer speaking and stacks of cash. “She funds her campaign with big money from big drug and big insurance-company executives. No wonder Gretchen Whitmer opposes single-payer health care.” To McCann, it seemed likely that allies of El-Sayed or Thanedar had paid for the ad, because it so exactly echoed the lines of their campaign.

On his phone, McCann took three screenshots of the ad, making sure to capture the final frame, which said, in small type, “Paid for by Priorities for Michigan.” McCann has been involved in Democratic politics in Michigan for almost twenty years, but he had never heard of the group. He texted a few other Democratic operatives. “No one had any clue,” he told me. He searched for Priorities for Michigan on the Michigan secretary of state’s Web site, which lists all campaign committees and PACs registered in the state, but found nothing. McCann tried Googling “Priorities for Michigan,” but that didn’t work. “Every politician in the state gives speeches about their ‘priorities for Michigan,’ and those were all the hits,” he said. Whitmer was being attacked for shadowy corporate ties by a group that itself left no trace.

McCann, searching for the source of the Priorities for Michigan ad, found another in the screenshots: the group had listed a post-office box in Lansing. He Googled it and got hit after hit for another political group, League of Our Own, which seemed to be promoting female candidates. (There was no mention of Priorities for Michigan on the active version of the Web site, but, on Google’s cached version, McCann found “Paid for by Priorities for Michigan,” along with the same post-office-box number.)