The practice is highly unusual on the federal level, and Waters appears to be the only national politician using it to grab committee cash. Slate mailers, however, are commonplace in her home state of California, though critics say it involves deceptive practices over one campaign paying another for a politician's backing in an election.

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Waters received the green light to run the operation from the Federal Election Commission in the fall of 2004. Since then, Karen Waters has received more than $1.2 million in payments from her mother's campaign.

The practice has also proved profitable for Waters' campaign. While most of the cash it garners is from individuals running for local public offices in the Golden State, state-wide politicians have also paid large sums to appear in the Waters mailers, including Vice President Kamala Harris. 

In 2010, when Harris sought the California attorney general spot, her campaign sent $33,000 to Waters' campaign for slate mailer placement. Later, in 2016, Harris paid $30,000 more to appear again while running for the U.S. Senate, filings show.

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Additionally, Democratic Gov. Gavin Newsom paid Waters $45,000 for the service while running for lieutenant governor in 2010, and former Sen. Barbara Boxer spent a combined $33,000 in 2010 and 2018 on the mailers. 

More recently, controversial Los Angeles District Attorney George Gascón dished out $36,500 to Waters' committee for the slate mailers during his 2020 campaign. 

Waters' campaign did not immediately return a Fox News Digital request for comment on the payments.