(WALL STREET JOURNAL) When Affordable Care Act insurance marketplaces launched in fall 2013, Arizona seemed like a success. Eight insurers competed to sign up consumers, offering a wide variety of plans and some of the lowest premiums in the country.

Today, with ACA enrollment starting Nov. 1, Arizonans will find in most counties only one insurer selling exchange plans for 2017. Premiums for some plans will be more than double this year, some of the biggest increases in the nation. Only last-minute maneuvering prevented one Arizona county from becoming the first in the nation to have no exchange insurers at all.

A similar dynamic is playing out in other states’ exchanges, which are a critical centerpiece of the 2010 health law. About one-third of U.S. counties will have just one exchange insurer next year, up from 7% this year, estimates the nonprofit Kaiser Family Foundation, which studies health-care issues. In many cases, remaining insurers are seeking significant rate increases.

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