Earlier this month, Julie Morringello, an artist in rural Maine, received a notice that her health care premiums could nearly double next year.

She now pays $460 a month for her "Obamacare" plan, but that amount is contingent on government subsidies that the Republican-controlled Congress may not extend.

"I don't know what we're going to do," Morringello, 58, said. Her insurance also covers her 14-year-old daughter, and forgoing a plan altogether isn't an option because Morringello has a history of cancer and needs continuing care.

Similar sticker shock may await millions of Americans who must start to sign up for coverage in November. The vast majority of people enrolled in plans under the Affordable Care Act receive additional federal tax credits that were first expanded by President

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