Obamacare's opening day drew millions of consumers to the law's core insurance exchanges, offering supporters and investors confidence that if the websites can stay up and running, customers will follow. In New York, officials said their exchange had 2.5 million visitors in its first half hour. California reported as many as 16,000 hits a second. And U.S. officials recorded 2.8 million visitors to the federal website, healthcare.gov, even as it fought technical problems much of the day. The difficulties with the online insurance marketplaces gave new ammunition to Republicans who say the Affordable Care Act doesn't work.