Ending the health law, even temporarily, “would wreak havoc in our health care system,” said Becerra in a call with reporters last week. “And we don't believe Americans are ready to see that their children are no longer able to see a doctor or that they cannot get treated for a preexisting health condition.”

Here are five questions and answers to help understand the case, Texas v. U.S.

1. What is this suit about?

In February, 18 GOP attorneys general and two GOP governors filed the suit in federal district court in the Northern District of Texas. They argue that because the Supreme Court upheld the ACA in 2012 by saying its requirement to carry insurance was a legitimate use of Congress' taxing power, eliminating the tax penalty for failure to have health insurance makes the entire law unconstitutional.

“Texans have known all along that Obamacare is unlawful and a divided Supreme Court's approval rested solely on the flimsy support of Congress' authority to tax,” Paxton said in a statement when the suit was filed. “Congress has now kicked that flimsy support from beneath the law.”

The lawsuit asks the judge to prohibit the federal government “from implementing, regulating, enforcing, or otherwise acting under the authority of the ACA.”

2. Why are Democratic attorneys general defending the law?

The defendant in the case is technically the Trump administration. But in June, the administration announced it would not fully defend the law in court.

The Justice Department, in its filing in the case, did not agree with the plaintiffs that eliminating the tax penalty should require that the entire law be struck down. But it did say that without the tax, the provisions of the law requiring insurance companies to sell to people with preexisting conditions and not charge them more should fall, beginning Jan. 1, 2019. That is when the tax penalty goes away.

The Republican attorneys general say they still believe the entire law should be invalidated, but if that does not happen, they would accept the elimination of the preexisting condition protections.

The Democratic attorneys general applied to “intervene” in the case to defend the law in its entirety. They say they needed to step forward to protect the health and well-being of their residents. The judge granted them that status on May 16.

3. What would happen if the judge grants a preliminary injunction?

The GOP plaintiffs say the law needs to be stopped immediately, “both because individuals will make insurance decisions during fall open-enrollment periods and because the States cannot turn their employee insurance plans and Medicaid operations on a dime,” according to their brief.

But setting aside the ACA while the case proceeds “would throw the entire [health] system into chaos,” Becerra said. That's because the ACA made major changes not just to the insurance market for individuals, but also to Medicare, Medicaid and the employer insurance market.

Even in 2012, when the Supreme Court was considering the constitutionality of the law before much of it had taken effect, some analysts from both parties predicted that finding the law unconstitutional could have serious repercussions for the Medicare program and the rest of the health care system.

In practice, however, even if Judge O'Connor were to rule in favor of the Republicans' request to stop the law's enforcement immediately, the decision could be quickly appealed up the line, including, if necessary, before the Supreme Court.

4. Is this case purely Republicans versus Democrats?

The case is largely partisan — with Republicans who oppose the health law arguing for its cancellation and Democrats who support it fighting to keep it in place.

But a friend-of-the-court brief filed by five law professors who disagree on the merits of the ACA said that, regardless, both the GOP states and the Justice Department are wrong to conclude that eliminating the tax penalty should result in the entire law being thrown out.

In this case, “Congress itself has essentially eliminated the provision in question and left the rest of a statute standing,” so courts do not need to guess whether lawmakers intended for the rest of the law to remain, they wrote.

5. What is Congress doing about this?

Technically, Congress is watching the case just as everyone else is. But Republicans in particular, while they mostly oppose the health law, are aware that the provisions protecting people with preexisting conditions are by far the most popular part of the ACA. And Democrats are already using the issue to hammer opponents in the upcoming midterm elections.

Last month, 10 GOP senators introduced legislation they said would maintain the ACA's preexisting condition protections in the event the lawsuit succeeds.

“This legislation is a common-sense solution that guarantees Americans with preexisting conditions will have health care coverage, regardless of how our judicial system rules on the future of Obamacare,” said Sen. Thom Tillis (R-N.C.), the bill's lead sponsor, in a statement.

Critics, however, were quick to point out that the bill doesn't actually offer the same protections that are embodied in the ACA. While the health law requires coverage for all conditions without extra premiums, the GOP bill would require that insurers sell to people with preexisting conditions, but not that those policies actually cover those conditions.