The Affordable Care Act is facing another challenge at the Supreme Court in King v. Burwell, which deals with subsidies for health insurance. The case could cut out a major provision of Obamacare, causing the law to unravel. Here’s what you need to know about the case. (Julie Percha/The Washington Post)

The outcome of the Supreme Court arguments about the new health-care law could turn on how to interpret a single hotly contested phrase in the massive bill. But the case has already highlighted this truism: Congress can sometimes be sloppy.

The justices on Wednesday hear arguments about federal subsidies that help millions of Americans afford insurance, with challengers arguing the Affordable Care Act, as written, only allows these payments for coverage bought through marketplaces set up by state governments. After all, the plaintiffs say, the bill says subsidies only apply in cases of “an Exchange established by the state.”

Get the latest updates from inside the Supreme Court here

But two-thirds of the states, mostly Republican led, have balked at setting up their own insurance marketplaces. So in these places, consumers buy coverage through an exchange set up by the federal government. The Obama administration and its allies say Congress clearly meant to include all Americans who qualified, regardless of whether they bought the insurance through a state exchange.

If that’s the case, how did Congress end up writing such an ambiguous provision? And why hasn’t anyone on Capitol Hill fixed it?

The U.S. Supreme Court will hear a challenge to President Obama’s health-care overhaul. (J. Scott Applewhite/AP)

It turns out, Congress often makes mistakes when writing laws.

In other instances, sometimes a few years later, Congress comes back with a “corrections bill” to fix mistakes that have become apparent. Sometimes lawmakers wait for the courts to tell them what they got wrong and then make amends after the fact.

“This happens all the time,” said Sen. John Cornyn (R-Tex.), the No. 2 Republican leader. “Just in terms of statutory construction, trying to figure out: What in the world did Congress mean?”

But partisan differences over Obamacare run so deep that there’s been no possibility of fine-tuning the law or winning agreement among lawmakers over what it actually means.

Cornyn and other Republicans who oppose the health-care law are hoping that a majority of the five justices take the “strict construction” view. “You can’t infer anything, but from the words on the paper. That’s the only thing Congress votes on,” he said.

Democrats reject that approach. “Our intent was clear,” Sen. Charles E. Schumer (D-N.Y.) said.

“This seemed like a non-serious court case,” Sen. Chris Murphy (D-Conn.) said, rejecting the idea that Democrats had erred in writing the bill. “There’s no reason to come back and amend this.”

Here’s what happening at the Supreme Court — and what comes next

If the Supreme Court invalidates the subsidies paid for insurance bought on the federal exchange, millions of Americans could see their costs soar and cancel their coverage, sparking further cost hikes and cancellations. This dynamic could ultimately undermine the financial viability of the entire health-care program. According to administration officials and their allies, this is certainly not what the ACA’s backers in Congress wanted — even if they didn’t say so explicitly.

While legislative drafts can often contain muddied language, the battle over Obamacare may represent the most significant legal challenge to a law based on what some might consider careless wording.

The issue of imprecise lawmaking came up recently in the context of the District of Columbia’s plan to legalize possession of marijuana. Congressional Republicans sought to impose restrictions on the plan by using a massive December bill to fund almost all of the federal government. They included a line forbidding the new marijuana measure from taking effect by including a line in legislation providing federal funds to the District.

After the fact, GOP lawmakers discovered that District officials had found a loophole to exploit. District officials declared that the restriction was null and void because the marijuana law had already been enacted when the voters approved it, and last week the law took effect, making it legal to smoke marijuana.

The issue seems destined for the federal courts — and also for a potential do-over by Republicans when they write funding legislation later this year.

Back in 1987, Congress had to rewrite a landmark budget-cutting bill, known as Gramm-Rudman-Hollings, that called for automatic spending cuts. A congressional Democrat sued, arguing that such mandatory cuts were unconstitutional because only Congress could dictate spending, and the Supreme Court invalidated the law.

The next year Congress passed what many dubbed “Gramm-Rudman-Hollings 2,” clarifying how the spending cuts would work.

Congress also had to rework its long-term highway bill drafted 10 years ago. The $300 billion bill also contained a litany of mistakes or issues that became out of date over time, leading to a “technical corrections” bill in 2008. Some funding levels for projects needed to be revised by small amounts — a transportation center went from receiving $2.225 million to $2.25 million — and others were reduced or nixed.

Such technical corrections have never been possible for the health-care law.

“Under different circumstances,” Cornyn said, “there might be some collective effort to offer a correction or a change to sustain it. But in this instance, because the parties are so polarized, because this was done as a purely partisan piece of legislation, it’s going to be up to the courts to decide.”

While the law had won a majority of votes in both the Senate and House, they were all Democratic votes. Cornyn argued that this closed the door to fixing its flaws.

“It is a mistake to do something on a purely partisan basis, because it’s not, ultimately, sustainable,” he said.