Washington - President Barack Obama's announcement of sweeping changes to the nation's immigration system is likely to lead to a battle over their legality. Is he on solid legal ground?

For months the White House and Obama's supporters have insisted that he has the authority to direct immigration authorities to exercise discretion in deciding which immigrants in the country illegally will face deportation and which won't.

But Republicans in Congress disagree with the White House view. They're calling Obama's plan an unconstitutional power grab.

Among those being protected from deportation under Obama's plan are the parents of U.S. citizens and legal permanent residents, roughly 4.1 million people.

A senior administration official said Thursday that the decision to protect this group is in line with existing law that allows adult citizens to sponsor their parents for immigration. Obama's plan goes a step further because the sponsoring citizen doesn't have to be an adult. And in such cases, the protection from deportation would be temporary - 3 years. The official spoke on condition of anonymity because he wasn't authorized to be identified by name.

At issue is how far Obama can go on his own to shield from deportation immigrants who are in the country illegally. The administration and its supporters have argued that the use of prosecutorial discretion - the ability to decide which cases will be pursued by prosecutors, either in immigration or criminal court - allows the president to decide which groups of immigrants should be a priority. Obama has argued that he can go one step further and use a provision in immigration law called "deferred action" to formally protect particular immigrants from deportation.

Immigrants granted deferred action are also eligible for work permits.

What the president can't do is halt all deportations, or permanently change the immigration status of any specific group of immigrants. Only Congress has that power.

While Obama's proposals have been cleared by lawyers from both the Homeland Security and Justice departments, some of the Republican governors meeting in Florida this week said they were weighing a lawsuit to block the president's action.

Outgoing Texas Gov. Rick Perry said a lawsuit was "very likely," and Indiana Gov. Mike Pence and Louisiana Gov. Bobby Jindal suggested they'd be willing to join the legal challenge.

"It should be immediately challenged in court, and we should seek an immediate stay," Pence said in an interview.

It's unclear how such a challenge would fare. A lawsuit challenging the 2012 program that protects many young immigrants from deportation was filed in federal court in Texas, but was dismissed on technical grounds.