Health care coverage has become more common in the United States than wearing a seatbelt, the AP reported earlier this year. The reason: 20 million Americans gained insurance under the Affordable Care Act, sending the uninsured rate plummeting to an all-time low.

But this might be about to change.

President-elect Trump and congressional Republicans have promised to repeal the Affordable Care Act — and early Thursday morning, the Senate took a key vote that kicks off that process.

Republicans say they will replace it with something much better. Economists, however, predict that anywhere from 3 to 21 million Americans will lose health coverage, depending on the plan.

And the fight over Obamacare extends beyond the coverage math. Under the law, fewer Americans are skipping doctors’ visits than before, for example. Most enrollees say they like their new coverage. But problems are real. The Affordable Care Act’s marketplaces have struggled to attract health insurers who want to sell coverage, creating serious questions about the sustainability of a major piece of the law. About half of Obamacare enrollees say that they’re unsatisfied with the high costs of premiums and deductibles.

These charts explain where Obamacare stands right now: who it covers, what is working and what needs a serious fix. They also show how Republicans envision changing the law, who the details would favor, and which people stand to lose coverage.

Millions of Americans have gained insurance coverage

1) The uninsured rate is at an all-time low. The federal government announced in September that 8.6 percent of Americans lacked health insurance. That’s a big decline from 2010, when the health care law passed and the uninsured rate was 16 percent.

2) Obamacare enrollees have generally been pretty happy with their health coverage

There are two major surveys of Obamacare enrollees, one run by the Commonwealth Fund and another by the nonprofit Kaiser Family Foundation. Both tend to show that Obamacare enrollees generally like their coverage.

The Commonwealth Fund’s most recent survey, conducted in 2016, showed that 82 percent of Medicaid and marketplace enrollees say they are “very satisfied” or “somewhat satisfied” with their plans. They found that most enrollees say they’re able to find a doctor who accepts their plan, and that their provider has space to see them within 14 days of signing up.

3) Obamacare enrollees’ biggest frustration remains affordability

This chart is from the Kaiser Family Foundation’s 2016 survey of marketplace enrollees. It shows that people who buy marketplace coverage are generally quite happy with their choice of hospitals and doctors. But the one area of the law that is the most consistent frustration is the high levels of out-of-pocket spending.

Forty-five percent of marketplace enrollees say they’re dissatisfied with the size of their deductible, and 40 percent with their premiums. The Kaiser Family Foundation estimates that, in 2016, the average deductible for a mid-level plan was $3,064 for an individual. That’s a significant increase from the average, $2,556 deductible for mid-level plans in 2015.

4) And the law has struggled to keep health insurers on board to sell coverage in the marketplaces

Before the election, one of the biggest questions swirling around the Affordable Care Act’s future was whether the insurance marketplaces were stable. In 2016, multiple large insurers, like Aetna and UnitedHealth, decided to stop selling health plans on the Obamacare marketplaces.

Much bigger swaths of the country had only one health insurance company selling in their area this year. Competition on Healthcare.gov hit an all-time low, as you can see in the chart above, which shows the percent of counties on Healthcare.gov with just one health insurance plan selling coverage.

Obamacare remains politically unpopular, with most Republicans supporting repeal

5) Obamacare remains quite divisive, and hasn’t gotten any more popular as Americans gained coverage

Every month, for nearly seven years now, the Kaiser Family Foundation has run a survey where they ask Americans if they have favorable or unfavorable opinions of the Affordable Care Act.

And one of the remarkable things about this study is that it shows very, very little change in public opinion on Obamacare. Democrats had a theory, back when they passed the law in 2010, that health reform would get more popular as the benefits rolled out and more people got coverage. "As that bill is enacted, it’s going to become more and more popular," Sen. Charles Schumer (D-NY) predicted on Meet the Press in March 2010.

But that change never happened — and the country remains just as divided on the health care law as it was on the day Obamacare passed.

6) Support for Obamacare repeal has hovered around one-quarter to one-third of voters for over a year now

Voters have also remained pretty evenly split on what the future of Obamacare should be. There is a nearly-even number of people who want to expand the law — and those who want to repeal it entirely. Support for Obamacare repeal did not decline as millions gained coverage.

Republicans plan to repeal Obamacare

7) Obamacare repeal, on its own, would leave an estimated 30 million Americans without health coverage

Republicans began to lay serious groundwork for Obamacare repeal last winter. In January, both the Senate and the House passed a reconciliation bill that took apart Obamacare’s expansion of Medicaid and private, subsidized health insurance.

The bill didn’t matter much at the time — Obama repealed it when it arrived at his desk — but it showed that Republicans could use the reconciliation process to take apart key Obamacare pillars, requiring a simple majority rather than the 60 votes needed to overcome a filibuster.

This is the type of bill that Republicans are likely to use when they pursue Obamacare repeal in early 2017. Congressional leaders have promised that, a few years down the line, they would follow up with a replacement plan that would provide coverage for many. If that didn’t happen, the Urban Institute estimates that repeal on its own would increase the number of Americans without insurance coverage by about 30 million in 2017. This would largely reflect a loss of coverage in both the individual market, as well as Medicaid.

8) There are 52 million people with preexisting conditions, and they would face more risks under the Republican plans

An estimated 52 million Americans — 27 percent of the non-elderly — have pre-existing conditions health conditions that, prior to the Affordable Care Act, would have been grounds for denying coverage in the individual market.

The health care law eliminated an insurance industry practice called underwriting, where plans would try to estimate how healthy their customers were to decide what price to charge — or whether to sell insurance at all. Obamacare regulations prohibit charging sicker people higher premiums; the only factors an insurance plan can take into account is where someone lives, how old they are, and if they smoke.

Most Republican plans, like Obamacare, require insurance plans to offer coverage to any patient regardless of how sick they are. But to be clear: This is not a repeal of preexisting conditions altogether. The Republican plans would let insurers charge sick people more if they did not maintain “continuous coverage.”

For example: If a cancer patient goes straight from insurance at work to her own policy, her insurer has to charge her a standard rate — it can’t take the cost of her condition into account.

If she had a lapse in coverage and then went to the individual market, under Better Way insurers would still have to offer her a plan — but it wouldn’t have to be affordable. And this is really different from Obamacare, which completely eliminates this type of insurance behavior. You can read more about this policy here.

9) Obamacare repeal would amount to a massive tax cut for the wealthy, too

The Affordable Care Act includes many new taxes, often levied on the highest-income Americans to help finance the expansion of health coverage for lower-income citizens. This includes a 3.8 percent tax on investment income over a certain threshold, as well as additional payroll taxes on high earners.

The Tax Policy Center, a nonpartisan nonprofit based in Washington that provides analysis of tax issues in policy, finds in a new report that the net effect of Obamacare repeal is a massive tax cut for the rich.

TPC estimates that the top 1 percent of earners would get an average tax cut of $33,000 if Obamacare is repealed — and those in the top 0.1 percent would get an average tax cut of $197,000.

Meanwhile, those who earn less would actually see their taxes, on average, go up. This is because most of the tax subsidies to purchase insurance coverage through the marketplace go to low-income Americans. TPC estimates that, on average, Americans who are in the bottom quarter of earners would see their taxes increase by $90.

Obamacare replacement plans would leave millions uncovered — and create new winners and losers

10) Republican replacement plans would cover millions, but not as many as Obamacare currently does

Republicans do have replacement plans for the Affordable Care Act. This includes the Better Way plan from House Speaker Paul Ryan (R-WI), the Patient CARE Act from Senate Finance Chair Sen. Orrin Hatch (R-UT), and a plan offered by President-elect Donald Trump during the election.

There is only a little bit of economic analysis available, so far, on how these plans would work. But the data that does exist shows that the Republican replacement plans raise the level of coverage in the United States from where it was pre-Obamacare, but leave millions who currently have coverage without a plan.

There is a lot of variation between Republican plans and how many people they cover. Trump’s plan repealed Obamacare and replaced it with very little in terms of coverage expansion. The plans from congressional leaders like Ryan and Hatch have much more robust plans for replacement, which include tax credits to buy insurance and maintaining some of the protections that Obamacare created for people who buy their own coverage. This gets them closer to the health law’s levels of coverage, but not all the way there.

11) Republican replacement plans want to change the rules about how much insurers could charge old and young consumers. This would lower premiums overall…

Obamacare’s marketplaces have struggled to attract young adults at the level the White House had initially hoped. The Obama administration originally said it wanted one-third of the marketplace to be people between 18 and 34 but, right now it’s only about a quarter.

Most Republican plans would try to attract younger people to the market by allowing insurers to charge them significantly lower premiums than the Affordable Care Act allows.

A bit of background information is useful here. Right now, insurers can only charge the oldest enrollees three times as much as the youngest. Obamacare drafters wanted to make it easier for older Americans, who often need more medical care, to find affordable coverage.

The Republican plans would change the individual market to allow insurers to charge the oldest enrollees five times as much as the youngest.

The nonpartisan RAND Corporation has modeled the effect of this switch. It found that, on average, premiums in the individual market would decline by $400 as a result of this change. This would be the result of more young people coming into the market and older people, who would face higher premiums, leaving it.

12) But there would be winners and losers. Young people would see their premiums go down, and older people would see their premiums go up.

Widening the age band, as this ratio is known, “increases the overall number of people with coverage, but older people end up falling out of the market as premiums rise,” says Christine Eibner, the RAND economist who has studied the provision.

You can see why older people might leave the market that Republicans set up in the numbers above. Premiums for the oldest enrollees would rise, on average, by nearly $2,000.

This worries some Obamacare supporters, who say the goal of insurance reform isn’t just expanding coverage — it’s expanding coverage to people who really need health coverage.

“If you replace a 60-year-old with a 20-year-old, that doesn’t change the number of people covered, but it changes the value of the coverage and of the program,” says Jonathan Gruber, the MIT economist who helped the White House model the economic effects of Obamacare. “You would see a shift in who is covered.”

13) Republican replacement plans would provide equal subsidies for everyone. Right now, lower income people get more financial help.

The Republican health plans, like Obamacare, envision that Americans will use tax credits to purchase individual health insurance, but the structure of the tax credits is very different in an important way.

Obamacare’s tax credits are based on income, with those who earn less getting more help. Better Way’s tax credits would only be based on age, giving more help to those who are older (and who will presumably be charged higher premiums).

This means that that Bill Gates would qualify for the largest tax credit simply because he is 61 years old. Conversely, a 23-year-old with little income and health problems gets minimal help, despite the fact that he needs support much more than Gates does.

We actually have quite specific numbers on how this would work from the Empowering Patients Bill put forward by Rep. Tom Price (R-GA), Trump’s pick to run Health and Human Services.

Above is data I looked up for what subsidies would look like for a 52-year-old buying coverage in Colorado Springs (I chose the area because I was giving a talk there recently). You can see that someone who earns more does better under the Price plan — but someone who has relatively low incomes could see their subsidy decline sharply.

How far will Republicans go on repeal?

14) Most Republicans support repeal, although the numbers did decline a bit after the election

The majority of Republicans favor Obamacare repeal, and have for quite some time now. This helps explain why legislators are moving so quickly on the issue, as they know it’s one that their base is pretty invested in seeing move forward.

One notable finding in recent polling, however, is that support for Obamacare repeal among Republicans has dropped since the election — suggesting that there might be some hesitancy to dismantle a law that provides health coverage to 20 million Americans. This is just one poll, however, and it’s unclear how much of the change is signal or noise.

15) Republicans’ leading strategy is “repeal and delay” — but that’s pretty risky

Republicans seem to be coalescing around a plan to repeal Obamacare — and set up a deadline, three years in the future, to pass a replacement plan.

The idea, politically, makes sense. Republicans can quickly make good on their promise to repeal Obamacare, and then get to the hard work of actually figuring out what should come next.

But the strategy is an especially risky one, especially when you consider the current state of the marketplaces. Repeal-and-delay would tell plans that sell on the Obamacare market that it is closing in a few years — a good reason for them to pack up shop and go home.

Obamacare can’t really afford to have many insurers leave. Even before the election, Healthcare.gov struggled to attract competition. For 2017 coverage, 40 percent of counties on the Healthcare.gov marketplace have just one health plan selling coverage.

“If we don’t know what the replacement plan looks like, many insurers might drop out of the marketplace or price very conservatively, charging a lot more because of the risk and uncertainty in the market,” says Topher Spiro, vice president for health policy at the left-leaning Center for American Progress.

Watch: Repealing Obamacare could change millions of lives