The sharp drop since then presents a dilemma for opponents of the ACA, including Republicans who control both houses of Congress and GOP presidential nominee Donald Trump.
Health-care analysts agree that a full repeal of the ACA, without any replacement plan, would lead to millions of newly insured people losing their health coverage. Such a wholesale purge of the coverage rolls would likely spark a political backlash against Republicans.
A glimpse of how bad that backlash could be came in late 2013, when several million Americans had their insurance plans canceled by insurers because the plans didn't comply with Obamacare's minimum standards.
Many of those people obtained new coverage under ACA-compliant plans, and experts agree that the number of people who lost their old plans was dwarfed by the numbers of people who became newly insured. But the Obama administration, which was stung by criticism that the cancellations contradicted President Barack Obama's promise that people would not lose their plans under the new law, quickly moved to grandfather plans at risk of cancellation to allow some people to remain on them.
Repeal of the ACA would lead to many more people actually losing coverage than the number of people who began receiving cancellations for plans in 2013.
An estimated 20 million or so people have gained coverage under the ACA's provisions since 2010. Those provisions include allowing adults under the age of 26 to stay on their parents' plans, expanding Medicaid eligibility to nearly all poor adults in 31 states that adopted expansion and selling subsidized health plans on Obamacare exchanges.
While Republicans including Trump have put forward proposals for replacing Obamacare with a GOP-designed health-care law, it's not clear if those proposals would come close to maintaining coverage for nearly all of the people covered by the ACA.
Another complicating factor for Republicans is that a number of GOP governors, despite their party's opposition to Obamacare, have expanded their individual states' Medicaid programs to cover all adults earning up to 138 percent of the poverty level. Those governors include Ohio's John Kasich, who ran for the party's presidential nomination this year, and Indiana's Mike Pence, who is Trump's running mate.
Census data shows that states that expanded Medicaid experienced bigger drops in their uninsured rates than states that did not. Repealing Obamacare in GOP-led states that have expanded Medicaid could, in turn, put elected Republican officials at risk of keeping their offices from voters upset at losing their coverage, and at risk of losing support from hospitals and other business interests that have backed Medicaid expansion.