Karen Tumulty / The Washington Post

WASHINGTON — As Washington entered a shutdown, President Barack Obama is betting that today will be remembered in history for what he believes cannot be stopped.

“The Affordable Care Act is moving forward. That funding is already in place. You can’t shut it down,” Obama warned Republicans as the final hours before the shutdown deadline ticked away.

The day has arrived when millions of uninsured Americans have their first chance to sign up for what the administration says will be high-quality, affordable health coverage, achieving what presidents of both parties sought unsuccessfully for more than 60 years.

The coming months and years will show whether the new health care law, known as Obamacare, lives up to its aspirations. Those who sign up now, for instance, will not begin to receive benefits until January.

It faces challenges that are both substantive and political, and a degree of difficulty that has no historical parallel.

Social Security and Medicare, programs whose main function is to make payments to and for those who are eligible, look almost simple compared with the system that will be put into place with the Affordable Care Act. Obamacare seeks to establish new health insurance marketplaces and transform how care is delivered, while giving states significant leeway in determining how that will be done.

Not only must the complicated operation work as Obama has said it will, but it must survive a continued assault by Republicans, who demanded repeal or delay of the law as the price of keeping the government open and the nation solvent.

They also are certain to seize on every technical glitch and misstep.

“I’ve never seen a law implemented with so many delays, mistakes and problems,” House Small Business Committee Chairman Sam Graves, R-Mo., said last week, after the administration announced that online enrollment in the new marketplaces for small businesses would not proceed on schedule.

The Treasury Department has also delayed by a year the requirement that businesses with 50 or more full-time employees provide coverage for their workers or face a fine — a move that the administration said was proof of its flexibility but one that opponents contended was an early warning of disaster.

Implementing a big new law “is not sexy. It’s not exciting. But it’s critically important, and it’s incredibly hard,” said Gautam Mukunda, author of the book “Indispensable: When Leaders Really Matter.”

In the case of the Affordable Care Act, implementation is made all the more difficult by how many moving parts are outside the president’s reach.

“He has significant but not total control over the federal bureaus; he has no control whatsoever over the states,” said Mukunda, who teaches at Harvard Business School. “But he’s going to get the blame for anything that goes wrong.”

“The president has to continue to focus,” said Robert Blendon, a professor of health policy and political analysis at Harvard University. He added that the launch of the new health care system “is a historic evolution of events. It is not a day.”