Iowa Republican Sen. Joni Ernst is running for reelection in what will likely be one of the nation’s most closely watched Senate contests this year. But her campaign has shielded a key bit of financial information from voters and the public: who she’s paying to run her re-election effort, and how much.

Through a subtle accounting maneuver, Ernst’s campaign has for months avoided reporting the names of the campaign staffers on her payroll, how many of those staffers there are, and the extent of payments they’ve received. Instead, the campaign appears to have grouped all of its employees together in large lump-sum payments to a staffing and human resources company it uses to administer its payroll.

It’s an arrangement that campaign finance experts say may have violated the spirit, and possibly the letter, of Federal Election Commission rules on the disclosure of payments to campaign staffers. The Ernst campaign disagrees, saying it fully complied with disclosure requirements, even as other Senate campaigns that employ the same payroll management firm have disclosed far more about the sums steered to their individual staffers.