Nancy-Ann DeParle was assistant to the president and deputy White House chief of staff for policy from February 2011 to January 2013. From March 2009 to February 2011, she was counselor to the president and director of the White House Office of Health Reform.

For more than 100 years, leaders from both parties struggled to bring affordable health care to all Americans. When President Obama took up the fight, many people predicted defeat. Three years, 34 repeal votes, one Supreme Court decision and a presidential election later, some are questioning whether government is capable of implementing the historic law. Concern is understandable: The Affordable Care Act (ACA) transforms a health-care system that accounts for one-sixth of the U.S. economy and is central to our lives. But recent history shows that big changes in health-care policy can be implemented.

The worries being voiced are familiar to those who follow health-care policy. They’re certainly familiar to me. I became the administrator of Medicare and Medicaid in 1997, just as Congress passed the most sweeping changes in Medicare’s history. The Balanced Budget Act of 1997 cut nearly $400 billion from providers such as doctors, hospitals and nursing homes; cracked down on waste and fraud; created a new Medicare HMO program; and launched the Children’s Health Insurance Program (CHIP) with a goal of covering millions of uninsured children.

Even members of Congress who voted for the law soon questioned whether it could work. Critics said that it was too big and complicated, that payment reductions would drive doctors and hospitals out of business and strand senior citizens without care, and that states would not cooperate in enrolling children in CHIP. Some predicted a Y2K crisis that would cripple government computers.

In fact, the law saved billions more than originally projected and extended the life of the Medicare “trust fund” by almost a quarter-century. We modernized payment systems and cut Medicare waste and fraud nearly in half. Millions of children in every state were insured thanks to CHIP.

A few years later, similar doubts surrounded the launch of two Republican initiatives: President George W. Bush’s Medicare prescription drug plan and Mitt Romney’s health-care reform in Massachusetts. Today, the Medicare prescription drug benefit helps millions of seniors, and the Massachusetts plan is a model for the nation.

Soon the Affordable Care Act will add a new chapter to this history of real-world success.

Critics say the law is complex. They are right. When Obama first took office, 51 million Americans were uninsured, premiums had more than doubled in the preceding decade, and insurers could deny coverage to those who needed it most. If easy solutions existed, someone would have found them long ago.

The ACA tackles these problems. It provides near-universal coverage by requiring everyone who can afford coverage to have it; expanding Medicaid for the poorest and subsidizing those who are low-income or whose employers do not provide coverage; and creating online “health insurance marketplaces” where consumers and small businesses can shop for private plans competing on price and quality.

Much has been made of the states that have not established marketplaces. Governors who chose to sit on the sidelines made the wrong call, but the federal government is ready to step in. Working with insurers, states and others, the Obama administration has been quietly testing the new system — tapping many of the officials who helped implement the Balanced Budget Act and the Medicare drug benefit — to ensure that it works. And although public discussion of the ACA has been contentious, I have been part of pragmatic, behind-the-scenes conversations with state officials and businesspeople invested in the law’s success.

Critics say the law fails to bring down health-care costs. A review of the facts is in order. If you get insurance at work, the new law will not disrupt your coverage or make it more expensive. And premiums will fall for the overwhelming majority of consumers in the new marketplaces as coverage expands, administrative costs are reduced and discrimination based on gender and age is curbed. In 2014 alone, 6 million Americans will get tax credits, making health insurance more affordable.

The law includes virtually every big idea put forward to promote efficiency, such as rewarding doctors and hospitals for their quality — not quantity — of care. These policies are making a difference: Over the past three years, health-care costs have grown at their lowest level in 52 years. Reforms such as the penalties to hospitals with too many preventable readmissions are reducing spending.

Skeptics ask whether consumers will buy insurance when the marketplaces open. A four-year partisan campaign to confuse the American people about “Obamacare” hasn’t helped the enrollment effort, and Congress has denied the Department of Health and Human Services the resources it needs to inform the American people about the law. Private organizations such as Enroll America are working to fill the gap. Insurance companies also have added incentives to compete for new customers.

Amid predictions of failure, reforms that were unthinkable a few years ago are already the law of the land. Just ask the 6 million young Americans insured through their parents’ plans, thanks to the ACA.

Large-scale change is never easy. But recent health-care reforms have outperformed their critics. When the Affordable Care Act does the same, Obamacare will have completed its journey from a pitched partisan battle to a national point of pride.