More than 16 million Americans have gained insurance coverage as a result of President Barack Obama's health care law, the administration said Monday as the White House prepares to commemorate the fifth anniversary of the law's signing. 

In releasing the latest estimates, Health and Human Services Secretary Sylvia M. Burwell called it "the largest reduction in the uninsured in four decades." 

Obama signed the Affordable Care Act on March 23, 2010, but it has been politically divisive from the start. 

Democrats hailed it as the culmination of decades of effort to guarantee health coverage for all Americans, including people with health problems who until then could have been turned away by insurance companies. Republicans called it government overreach, and haven't stopped trying to repeal or roll back what they dismiss as "Obamacare." 

Burwell said the administration now estimates that 16.4 million people have gained coverage as a result of the law. 

The program offers subsidized private coverage for people who don't have health insurance on the job, along with an expanded Medicaid program that a majority of states have accepted. 

Most of those gaining coverage -- 14.1 million adults -- got their insurance after the law's big expansion began at the end of 2013. 

Another 2.3 million people had gained coverage previously. Those were young adults allowed to remain on a parent's plan until age 26 under one of the law's most popular provisions. 

Independent studies, including the extensive Gallup-Healthways Well-Being Index, have documented the same general trend. 

The administration said all racial and ethnic groups have seen gains in coverage, but the biggest improvement has come among minority groups. 

Since the start of last year, the uninsured rate dropped by more than 12 percentage points among Hispanics, more than 9 percentage points among African-Americans and more than 5 percentage points among whites. 

The coverage gains haven't settled the political debate. Republicans now in charge of both chambers of Congress remain committed to repeal, although Obama is sure to veto any such legislation. 

The biggest question hanging over the health care law now is a case before the Supreme Court, in which its opponents have argued that subsidies are illegal in most states. They contend that the exact wording of the law only allows subsidized coverage in states that have set up their own insurance markets, and most have not done so. The administration counters that the context of the law makes it clear its purpose was to expand coverage in every state. 

Independent estimates say about 8 million people could lose coverage if the subsidies are struck down. A decision is expected by the end of June.