WASHINGTON -- Less than a year before Americans will be required to have insurance under President Barack Obama's healthcare law, many of its backers are growing increasingly anxious that premiums could jump, driven up by the legislation itself.Higher premiums could undermine a core promise of the Affordable Care Act: to make basic health protections available to all Americans for the first time. Major rate increases also threaten to cause a backlash just as the law is supposed to deliver many key benefits Obama promised when he signed it in 2010."The single biggest issue we face now is affordability," said Jill Zorn, senior program officer at the Universal Health Care Foundation of Connecticut, a consumer advocacy group that championed the new law.Administration officials have consistently downplayed the specter of rate increases and other disruptions as millions of Americans move into overhauled insurance markets in 2014. They cite provisions in the law that they say will hold down premiums, including new competitive markets they believe will make insurers offer competitive rates.Exactly how high the premiums may go won't be known until later this year. But already, officials in states that support the law have sounded warnings that some people -- mostly those who are young and do not receive coverage through their work -- may see considerably higher prices than expected.That is because of new requirements in the law aimed at making insurance more comprehensive and more affordable for older, sicker consumers.Oregon's insurance commissioner, a supporter of the law, said new regulations could push up premiums for young customers by as much as 30 percent next year. He urged administration officials to slow enactment of the new rules.