The move puts the ACA in jeopardy once again as it faces potential repeal.
Trump tried to go further, pledging to replace the law with a new Republican plan before the 2020 election but beat a hasty retreat after his own party rebuked the idea.
Obamacare, which was signed into law in 2010, includes provisions that protect people with pre-existing conditions, expand Medicaid in most states and allow children to remain on their parents' insurance plans until age 26. The law's individual mandate required almost every American to purchase insurance or face a tax penalty.
The Trump administration reduced the individual mandate to $0 in the Tax Cuts and Jobs Act of 2017, a move that could lead to the law's undoing.
In December, a federal judge in Texas ruled Obamacare unconstitutional after 20 Republican-led states filed a suit against the health-care policy. U.S. District Judge Reed O'Connor said that without the individual mandate, the law could not stand.
O'Connor's ruling, backed by the Justice Department, awaits deliberation with the U.S. Court of Appeals for the 5th Circuit in New Orleans. But what does that mean for the millions of Americans who receive coverage under the Affordable Care Act?
Americans covered under the Affordable Care Act don't have to worry about losing benefits for now.
O'Connor said his ruling should not be immediately implemented because "many everyday Americans would otherwise face great uncertainty" regarding their health care during an appeal. A ruling from the 5th Circuit on O'Connor's decision is not assured for 2019.
Joseph Antos, a health-care and retirement policy scholar at the conservative American Enterprise Institute, said if the appeals court supports O'Connor's ruling — which he believes is unlikely — the earliest the Supreme Court would hear the case is sometime in 2020.
Antos added that people will also "undoubtedly" be able to stay on their Obamacare plans and receive subsidies in 2020 because enrollment for next year opens this fall.
Obamacare sign-ups for 2019 fell to 11.4 million people, down from 11.8 million the year before, after the Trump administration made changes to the law that experts blame for the decrease. Trump removed the individual mandate, cut the ACA advertising budget, allowed healthy consumers to stay on "skimpy" insurance plans and shortened the enrollment period for consumers to sign up.
Enrollment has been slowly decreasing since its peak of 12.6 million sign-ups in 2016.
Still, about 30 million people, either covered by Medicaid expansion or plans sold on the Obamacare exchanges, would lose their health insurance if the Affordable Care Act is repealed with no replacement plan.
The law, which sought to expand access to insurance, increase consumer protections and reduce health-care costs, initially required most Americans to purchase health insurance. If nonexempt Americans weren't covered, they faced a tax penalty of $695 per adult or 2.5 percent of household income, whichever was higher. The idea was to force younger, healthier consumers to sign up to mitigate the costs of covering older and sicker Americans. The mandate incensed conservatives who viewed it as a symbol of government overreach.
Consumers can now opt out of purchasing health insurance without facing a penalty since the Trump administration reduced the individual mandate to $0. But the Obamacare exchanges are still available to the public. Consumers in all 50 states and the District of Columbia have access to exchanges, either federal or state run.
Twenty-eight states use federally run marketplaces, while 12 states use their own state-based marketplaces. Eleven states use either federally supported state marketplaces or marketplaces that are run by a partnership between the federal government and the state.
In 2017, insurance companies began to abandon the Obamacare exchanges as they worried about potential changes a Republican administration would make to the Affordable Care Act. But insurers have since started offering plans in more states or expanding in states they already operate in as they started making profits in the individual market, giving customers more options.
Obamacare also expanded Medicaid eligibility to uninsured children or adults at or below 138 percent of the federal poverty level. It also made many people with mental illnesses eligible for coverage.
According to the Kaiser Family Foundation, 36 states and D.C. have adopted Medicaid expansion and 14 states have not. Medicaid enrollment increased by 16 million people since Obamacare went into effect, with 13.6 million living in Medicaid expansion states.
Critics of Medicaid expansion claim it is a financial burden on states, which have budgets that are already dominated by the cost of Medicaid. The federal government initially covered all the costs of expansion, but states were eventually expected to pick up 10 percent of the tab.
Though Trump made it clear he seeks to repeal Obamacare and replace it with a Republican alternative, he has yet to put forth a new plan.
Trump said in a late March tweet that the "Republican Party will become the Party of Great HealthCare," calling Obamacare a "disaster" because of high premiums and deductibles.
Insurers that publicly quantify a rate impact from regulatory changes reported an average increase in premiums (a monthly fee paid to insurance companies) of 6 percent after the individual mandate was repealed, the Kaiser Family Foundation said. Also, in December experts told NBC News that though deductibles are high, it's a problem for the entire health-care industry and not just Obamacare.
Despite his criticism, the president was forced to back away from his threat after his party expressed no appetite for revisiting the health-care debate.
Senate Majority Leader Mitch McConnell told Trump last week that the Senate would wait until Republicans gained full control of Congress before revisiting health-care reform. The GOP holds 197 seats in the House after Democrats regained control in the 2018 midterm elections, meaning Republicans would need 21 more seats to win a majority in the next election.
Trump then tweeted that Republicans are working on "a really great HealthCare Plan with far lower premiums (cost) & deductibles than ObamaCare," which would not be voted on until 2021 "when Republicans hold the Senate & win back the House."
The White House did not immediately respond to CNBC's request for comment.