April 8 (Bloomberg) — WellPoint Inc., the second-largest U.S. health insurer, is leading companies that have poured $13.4 million into defeating a ballot initiative that would give California regulators the power to reject increases in health policy premiums.

WellPoint, based in Indianapolis, gave $12.5 million and its Anthem Blue Cross unit gave $270,000 through April 2 to defeat the measure, according to data provided by MapLight, a Berkeley-based nonpartisan research organization. Oakland, California-based Kaiser Foundation Health Plan Inc. also gave $270,000. Supporters, led by Santa Monica-based Consumer Watchdog, have raised $425,521.

Health insurers stand to lose control over raising rates in the most populous U.S. state at a time of dramatic growth in the customer base. More than 1.2 million people have enrolled in private health insurance plans under Obamacare through the end of March, according to Covered California, the state's health- insurance exchange.