CHICAGO — Millions of Americans today will be able to shop for the first time on the insurance marketplaces at the heart of President Barack Obama’s health care reforms, entering a world intended to simplify the mysteries of health coverage but that could cause yet more confusion — at least initially.

Whether consumers will be pleased with the experience, the premiums and the out-of-pocket costs of the plans offered to them will finally start to become clear today. The nationwide rollout comes after months of buildup in which the marketplaces, also known as exchanges, have been both praised and vilified.

Illustrating the heated political disagreements over the law, the opening of the exchanges comes the same day as the shutdown of the federal government, led by congressional Republicans who want to block the health insurance reforms from taking effect.

The shutdown will have no immediate effect on the insurance marketplaces that are the backbone of the law, because they operate with money that isn’t subject to the annual budget wrangling in Washington.

The marketplaces represent a turning point in the nation’s approach to health care, the biggest expansion in nearly 50 years.

The Obama administration hopes to sign up 7 million people during the first year and aims to eventually sign up at least half of the nearly 50 million uninsured Americans through an expansion of Medicaid or government-subsidized plans.

But if people become frustrated with predicted glitches in the computer-based enrollment process and turn away from the program, the prospects for Obama’s signature domestic policy achievement could dim.

“The promise of the law is that no one will go bankrupt because of medical bills,” said Neera Tanden, president of the Center for American Progress. “It won’t happen in the first day or the first year. But when the law is fully operational, it will provide an economic benefit to roughly 30 million Americans.”

Tanden cautioned against rushing to judge the marketplace’s success on its first-day performance. Numerous observers have predicted bugs and setbacks. Trained outreach workers in many states are having trouble getting the certification they need to start helping people to enroll.

Many states are predicting people will initially test the online application system but actually sign up closer to Dec. 15, which is the deadline for coverage to start Jan. 1.

Under the law, health insurance companies can no longer deny coverage to someone with a pre-existing medical condition and cannot impose lifetime caps on coverage. They also must cover a list of essential services, ranging from mental health treatment to maternity services.