According to the NLPC, Waters' campaign sent out a "slate mailer" that included former Los Angeles Mayor Antonio Villaraigosa who was running for Governor in the Democratic primary. Villaraigoa's campaign agreed to pay a $25,000 fee for the mailer. NLPC says Villaraigosa failed to pay for the mailer but a group called "Families and Teachers for Antonio" paid the fee.
"Whereas candidates like Villaraigosa may legally pay Waters' campaign for the proportional costs of their inclusion on her slate mailer, it is not legal for such payment to be made by a third party like 'Families and Teachers for Antonio,'" NPLC alleges.
NLPC filed a similar complaint with the FEC for another "slate mailer" which was part of the 2016 election. The watchdog group says Waters' campaign accepted $35,000 from the Democratic State Central Committee of California (DSCCC) to include then-Senate candidate Kamala Harris.
Waters has been sending out "slate mailers," which resemble a sample ballot distributed by political parties before an election, for years.
According to the FEC's 2004 advisory opinion, payments from other candidates "would not constitute support of, or in-kind contributions to, any federal candidate appearing in the brochure, so long as the authorized committee of that federal candidate reimburses the Waters Committee."
Just my guess.