Buying health insurance will be as easy as purchasing a plane ticket or shopping on Amazon, President Barack Obama has promised.

Maybe, but perhaps not on Tuesday — the day when millions of Americans are supposed to be able to start buying coverage under the sweeping law formally known as the Affordable Care Act but often referred to as "Obamacare" by critics.

Widespread reports of computer problems and logistical glitches are casting a pall over what many supporters envisioned will be a triumphant day for the embattled program. State and federal health officials have said in recent days that some key functions of the online insurance marketplaces will not be ready right away. Some of the consumer guides meant to help people sign up for coverage are not yet certified to do so.