The Los Angeles city attorney launched a wide-ranging legal action on Thursday against one of California’s biggest health insurance providers, accusing it of illegally selling policies to customers and then denying them coverage when they fell ill.

The action is being launched as health insurers face closer scrutiny from US regulators and legislators. Health insurance has become one of the dominant themes of the presidential election, with Democratic challengers Hillary Clinton and Barack Obama promising to reform what they contend is a broken system.