Longtime Democratic lawmaker Maxine Waters has perfected an unusual tactic for fundraising over the years – getting candidates, including some of California’s most prominent political figures, running for state and local offices, to pay as much as $45,000 for her endorsement on election mailers. In this election cycle alone, Waters has raised 59 percent of her campaign’s treasury through these “slate mailers.”

The Los Angeles-area representative, who faces ethics charges in the House for intervening on behalf of a bank in which her husband had invested heavily, has found a way to take large sums of money from ...