What to Know About the Big Immigration Case at the Supreme Court

— -- The nation's highest court will hear arguments today in the case of the United States v. Texas.

Texas -- along with a number of other states -- is challenging President Obama's effort to, among other things, allow millions of undocumented immigrants to remain in the country.

A lower court blocked the action, which was announced in 2014, so the programs have not taken effect and their future remains in limbo as the Supreme Court hears the case.

It's a complex issue with very real human and political implications.

So Kate Shaw, an ABC News contributor and an assistant professor of law at the Benjamin N. Cardozo School of Law in New York, will help explain what you need to know.

1) Let's set the stage. What presidential action is the plaintiff, aka "Texas," challenging? What did Obama basically intend to do? Who would be affected?

SHAW: This case involves the administration’s 2014 announcement that it intends to grant “deferred action” — essentially, temporary relief from the threat of deportation — to millions of people living in the United States without legal status. The program would mainly apply to parents of U.S. citizens or lawful permanent residents. (That’s why it’s commonly known as “DAPA” – “Deferred Action for Parents of Americans and Lawful Permanent Residents.”)

But the announcement also expands an earlier deferred-action initiative that applies to people who came to the United States as children (that one is referred to as “DACA” – “Deferred Action for Childhood Arrivals”).

The administration argues that recipients of “deferred action” don’t receive lawful immigration status; they’re just notified that they’re not a deportation priority, so they can, in the administration’s words, “come out of the shadows,” and do things like apply for work authorization. Most estimates place the number of potentially affected individuals at 4 million or more.

2) What’s the core of the challenge to this plan? And how does Obama defend it?

SHAW: The states, joined by the House of Representatives (which will share argument time today), make a few arguments. They claim that the plan conflicts with the existing immigration statutes, unilaterally granting legal status to individuals who are here unlawfully under existing immigration law; they also argue that the administration should have announced the new policy through a formal “notice-and-comment” rulemaking process. And, after the Supreme Court specifically asked them to address this question, they argue that the plan is unconstitutional; that by crafting this plan the president has failed to “Take Care that the laws be faithfully executed.”

The administration responds, first, that Texas has no right to be in court attacking this policy in the first place. It also argues that the plan is well within the discretion Congress has granted the executive branch to set immigration priorities, and that the administration is just deciding how best to use its limited enforcement resources; that it was entitled to use the processes it did to announce the program; and that it is plainly constitutional.