Here's some of what's at stake as the U.S. Supreme Court weighs the future of Obamacare

When the U.S. Supreme Court begins weighing the fate Tuesday of the Affordable Care Act — commonly known as Obamacare for the president who passed it — arguments will revolve around arcane points of law, such as severability, specifically, whether the justices can surgically strike down part of the law and leave the rest.

But what's at stake has real-world consequences for just about every American and the health care industry. 

The argument against the law from the Trump administration and conservative states is that the 10-year-old legislation was rendered unconstitutional in its entirety when Congress eliminated a penalty on those remaining uninsured, known as the individual mandate.

Here's a look at some of what's at stake if the law is deemed unconstitutional by a court that now includes three justices appointed under the Trump administration.

Coverage for pre-existing conditions

Before the ACA, insurers could turn down a person for an individual policy or charge more based on medical history. The nonpartisan Kaiser Family Foundation estimates that about 54 million working-age adults have health issues that would have made them "uninsurable" before former president Barack Obama's signature law was passed.

Tens of millions more have issues that could have led to higher premiums. Women were also routinely charged more by insurers, it found.

Democrats fear COVID-19 could become a pre-existing condition insurers won't cover if the law is struck down, impacting some of the more than 10 million people who have tested positive for coronavirus, which causes the illness.

U.S. President Donald Trump has repeatedly promised to always protect people with pre-existing conditions but has not specified how.

Coverage for more than 20 million

Two main programs for covering uninsured people would be eliminated if the law is overturned, leaving more than 20 million people uninsured unless a divided Congress can put a new safety net in place.

About 12 million low-income people are covered through the health law's Medicaid expansion, now available in most states. Most of them are adults working in jobs that don't come with health insurance. More than 11 million people have private coverage purchased through taxpayer-subsidized private markets such as HealthCare.gov.

Preventative health services

Most American women now pay nothing out of their own pockets for birth control. That's covered as a preventive service under the ACA.

Some other services, such as colonoscopies and flu shots, are also free.

Democrats have argued that if co-pays for routine preventive care are reinstituted, that may discourage some people from getting tests that have been shown to detect diseases such as cancer at early stages when they are easier to treat.

Return of a Medicare gap

Obamacare took the first major steps to close Medicare's unpopular "doughnut hole," a coverage gap that left seniors on the hook for hundreds of dollars in prescriptions drug costs. Congress later accelerated the timetable to eliminate the gap.

Repealing the ACA could mean the return of the coverage gap, sure to infuriate older voters, many of whom say their medications still cost too much.

There could be other consequences for Medicare. For example, the ACA also slowed payments to hospitals and insurers to extend the life of the Medicare trust fund.

Coverage for dependents

One of the earliest benefits to take effect after the passage of Obamacare was a requirement that insurers allow young adults to stay on a parent's plan until they turned 26.

Before that coverage extension, insurers routinely cut off young adults at age 19 or 22 if they were full-time students.

What would come next

Passing the 900-page-plus ACA was a political challenge that took more than a year at a time when Democrats controlled the White House and both chambers in Congress.

Putting together a replacement under a divided government would be the ultimate political puzzle. Neither Democrats nor Republicans agree even within their own ranks what that should look like.

President-elect Joe Biden has said he would build on the ACA by improving it and adding a new public health insurance option. But some more liberal-leaning members of his party want a government-run system for all Americans, including the 160 million covered through employer plans.

Republicans, meanwhile, want to scale back the government's support for health care. They have said they would make cuts to Medicaid financing and leave the ACA's insurance markets as a state option. 

Trump once famously said, "nobody knew health care could be so complicated." That was in 2017, when he and a Republican-controlled Congress harboured hopes they could "repeal and replace" the ACA.

It didn't happen then because Republicans could not agree on what a replacement would look like.